The abovementioned fictional race aren't exactly merpeople from fantasy, but I call them like that just for the sake of convenience. Rather, they're semi-aquatic humanoids, who are really good at diving. This species still possess legs like humans, and they spend most of the time on their giant biological ships — which serve to the merfolk as mobile homes and fortresses. However, they almost never step on land (they don't even need to) except when raiding human coastal settlements and cities.

These bioships are basically domesticated "kaiju", looking like a cross between a turtle, a whale and a dragon. Their average length vary within 300-400 feet, they can reach speeds of approximately 30 knots and dive to more than 1000 feet under the surface for a long time. Like tortoises, these bioships have large bulletproof carapaces under which there are large cavities filled with air — where the merfolk live most of the time. From there the "crew" controls their living ships by utilizing some kind of psychic link with the bioship's mind.

There are dozens, if not hundreds of such kaijus, roaming the oceans. Many of the tribes consist of only one bioship, some unite in small fleets. But still most are divided and the tribes constantly fight between each other for resources, fishing territories, slaves and etc.

When a bioship's crew encounters an enemy (be it a hostile kaiju or a regular ship built of wood or metal), the tribe sends a "cavalry" (warriors riding dolphin-like animals) to deal with the threat, while the home carrier tries to hide. The warriors, armed with various bladed weapons, are accompanied by a dozens or even hundreds of specially bred fish, designed for suicide attacks by exploding themselves with highly corrosive acids capable of eating steel some kind of biological explosives — picture crashfish from Subnautica, but much deadlier. While the riders distract hostile ship's protectors (if the enemy is an another merfolk tribe), the suicide fish try to get close to the enemy's ship and breach its hull/carapace, thus incapacitating ship. However, if the enemies appear to be stronger, the entire tribe simply flees the battle.

For some reason, humans and merfolk didn't really encounter each other until this world's mankind reached 1940-1950s technology level. At this point merfolk started pillaging coastal towns and cities much more frequently. Naturally, the government couldn't ignore such a threat to its national security and began to dispatch military fleets for patrolling. However, because merfolk tribes do not possess any serious firepower, compared to conventional WW2 ships, they usually resort to hit-and-run tactics, fleeing away, once patrolling destroyers show up on the horizon. Alternatively, if the pillagers feel lucky, they might try to sneak upon the military ships in order to unleash acid fish on them.

And here is the question. I am by no means an expert on the naval tactics and technologies, so I would like to ask you for advice. What would admirals likely do in this situation, if the goal is to provide security for the vulnerable regions? Is there some way to track kaiju-ships in the open ocean? Or maybe it's better to reinforce static defences at the coastal cities, rather than trying to find a needle in a haystack? Anyway, an expert opinion from you is much appreciated!

P.S. One of the most likely strategies would probably involve hiring some tribes as mercenaries. Land humans have invented a lot of things that merfolk can't produce. Firearms, armor, medications, electronics, you name it. By bribing some of these pirates, humans can incite more infighting among warring tribes, thus reducing the threat to their cities to a more manageable level.

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    $\begingroup$ This question specifically asks about the action of a character and would be off-topic on this site. It would be very difficult to answer since you've not even specified the motivation (eradicate, tame, trade, capture, eat etc.). Please take our tour and have a read of our help center regarding our ways. Welcome to worldbuilding. $\endgroup$ Sep 14 '21 at 8:42
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    $\begingroup$ Real world acids, even the strongest ones, act slowly, and (as it is already pointed out in the answers) quickly dissipate in water. Should we take this as "magic acid", or "reality-check" or "science-based" tags are applicable for this question? $\endgroup$
    – Alexander
    Sep 14 '21 at 16:04
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    $\begingroup$ "The warriors, armed with various bladed weapons, ..." promptly get shot by the local police when they try to pillage a coastal town. It's unclear how they would carry away their loot anyway unless their kaiju surfaces, in which case en.wikipedia.org/wiki/Coastal_artillery is brought to bear. $\endgroup$ Sep 14 '21 at 16:07
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    $\begingroup$ Also, early 40s and late 50s were very different. They maybe just a few years apart, but many totally game-changing developments happened in those few years. Radar and sonar were very early technologies in the 40s and planes had propellers, while by late 50s you had ICBMs, nuclear subs and ships, jets, use of electronic computers, and many other things. $\endgroup$
    – Withadel
    Sep 15 '21 at 5:37
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    $\begingroup$ A detail that might make this work: pre- and early-WW2 navies had significant difficulties fighting at night. If you give the merfolk better night vision they stand a chance with sneak attacks. But really this setting would work much better in the whaling era of the Age of Sail. $\endgroup$
    – pjc50
    Sep 15 '21 at 9:39

The warriors, armed with various bladed weapons,

...are going to get absolutely wrecked by modern military firepower. Hell, they'd get wrecked by civilian firepower.

are accompanied by a specially bred fish, designed for suicide attacks by exploding themselves with highly corrosive acids, capable of eating steel

Trying to promptly dissolve tough materials that are immersed in the ocean is not a very valid tactic. If you filled the boat with enough acid from the inside it would eventually go down, but you'd need to kill everyone on board first to stop them hosing it down and pumping it out whilst they sail for shore.

Is there some way to track kaiju-ships in the open ocean?

ASDIC, which was the predecessor to SONAR, started development in 1916. This is a comparatively short-ranged system compared to the scale of an actual ocean, but without clarification is isn't obvious how much of the ocean would actually need to be surveilled... biology is complex, and you can't easily make something that can go shoplifting in San Juan in the morning and be back at the bottom of the Puerto Rico trench for tea time... carbohydrates and lipids in cells have different responses to pressure, and a biochemistry that works under a few thousand metres of water ain't gonna work so well in the open air.

Obviously, "kaiju" implies magic, but as you haven't defined or constrained it it is difficult to make any further suggestions as to what could be done.

Or maybe it's better to reinforce static defences at the coastal cities, rather than trying to find a needle in a haystack?

Against knife-wielding surprise night-time raiders, WW2 era pillbox-and-machine-gun coastal defenses will be brutally effective.

Against giant magical monsters... well, you haven't defined what they can do. Godzilla was magic and effectively indestructible. Things made of actual biology are going to suffer extremely badly from the effects of anti-tank weaponry on the surface, and and-submarine weaponry below it.

What would admirals likely do in this situation, if the goal is to provide security for the vulnerable?

The army and marines and local militia would secure the shoreline, and any spear-waving primitive that tries to go up against them will be shot dead.

Any kaiju detected coming in-shore are going to be hit by antisubmarine weaponry up to and possibly including tactical nuclear depth charges, and they are going to be liquified.

So either your merpeeps have magical indestructible kaiju, in which case you get to decide what can be done against them, or they have regular biology and what they have to do is to make peace as soon as they can, before technology gets good enough to hunt them down in the open ocean and nuke them.

an expert opinion from you is much appreciated!

Whilst it may be that there are some serious littoral warfare tacticians and strategists on stack exchange, I wouldn't hold up too much hope of getting their expertise here, especially against magical monsters ;-)

Land humans have invented a lot of things that merfolk can't produce. Firearms, armor, medications, electronics, you name it

For an underwater civilisation, almost all those things will be as useful as a chocolate teapot.

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    $\begingroup$ The other interesting aspect of active ASDIC/sonar in this context is that the intense blast of acoustic underwater energy it emits is reported to injure marine mammals, which presumably includes merfolk. $\endgroup$ Sep 14 '21 at 16:20
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    $\begingroup$ @GrumpyYoungMan I'm not sure when these devices became powerful enough to be injurious... I'm guessing mid to late cold war, but I could be wrong. Use of such devices (or things like having a war around the world with massive naval conflict followed by nuclear testing in the sea) might have provoked the merpeeps in the first place, of course. $\endgroup$ Sep 14 '21 at 18:35
  • $\begingroup$ @GrumpyYoungMan I don't think ASDIC "blasted" anything; it was a listening device. $\endgroup$
    – JDługosz
    Sep 14 '21 at 23:24
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    $\begingroup$ @JDługosz A casual check seems to indicate ASDIC was active sonar (e.g. "ASDIC is the original British name for the equipment which sent sound pulses on specific bearings in order to detect submerged submarines." from jproc.ca/haida/asdic_sonar_sys.html ) . According to britannica.com/technology/sonar , "An early passive system, consisting of towed lines of microphones, was used to detect submarines by 1916, and by 1918 an operational active system had been built by British and U.S. scientists.", which also corroborates active sonar tech being available by WW2. $\endgroup$ Sep 15 '21 at 1:24
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    $\begingroup$ "Things made of actual biology are going to suffer extremely badly from the effects of anti-tank weaponry on the surface, and and-submarine weaponry below it." Not to mention the square-cube law. $\endgroup$ Sep 15 '21 at 3:02

They would probably be at an initial disadvantage, unless they had a submarine war like our world

The dominant naval theory in our world was the Mahanian doctrine. This held that naval raiding couldn't win a war, and that decisive land bound ships were what mattered. At lower technology levels this is obviously true. Ships are very powerful, and did win many empires control of the seas. But, the 1940s to 1950s is where the inflection point happens, and submarines start to become more decisive weapons of war.

Germany proved that in ww2, with their aggressive submarine warfare. These naval nations, if not well prepared, could lose a lot. You need advanced radar and sonar technology to detect submarines, and it takes a lot of experience and advancement to make it a reliable weapon.

If the mermaid tribes realize the threat of humans they could launch massive assaults, using night attacks to launch dozens of pearl harbor scenarios.

Humans would advance technology and use convoys more

Radar and sonar technology would be a priority. If you improve them enough you can detect sea enemies. The enemy may have some skill with stealth, but humanity should be able to push past it with time.

Convoys would be used for ships. If you have enough ships you can present an overwhelming threat.

Harbours and coastal cities would try to fortify. You can't fortify every area though, so lots of use of people with binoculars and airplanes and fast moving ships would be used to defend coastlines. A lot of effort would be made to make secure communication lines so that you could launch quicker strikes.

A lot of nations would fail. These efforts require trust and good tech maintenance and very rapid responses. Some nations lack enough trust to be that agile.

The deep oceans would be hard to manage

Enough planes and ships and you can watch a huge region, but the ocean is very huge. The mermaids could hide from radar and sonar with that much area. You need much more advanced technology to search the ocean in depth, and even today we're not quite there. Submarines are still able to hide with nuclear devices in the deep oceans.

By the 2030s we'll probably be able to do it, with massive drone swarms, but we're not there yet.

  • $\begingroup$ To the convoy point. I would imagine it would be a mandatory tactic. We don't have a lot of details about the appearance or biology of these kaiju. But I would imagine that one of the most effective offensive tactics would be to surface underneath the ships and attempt to knock them over or ram them. Ships would need to travel in convoys to deter those types of attacks. In addition, solo sea travel would become very dangerous. $\endgroup$
    – SpencerG
    Sep 14 '21 at 18:40
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    $\begingroup$ Or you could just blast the water with low frequency sonar. That beached hundreds of whales in real life. $\endgroup$
    – DKNguyen
    Sep 14 '21 at 18:58

Submarines and aircraft carriers

WWII brought many ways of warfare into the spotlight, changing how we fight.

Submarines were plenty. Sonar might not be the best way to target other submersibles with much of the technology at that time but as the enemy has a floating/submersible city, targeting shouldn't be too difficult. Fire enough explosives under water, which are generally more powerful but with less reach and you can take those things down.

Next is airplanes. With patrols you can have reconnaissance over huge areas. Bombers and fighters have good firepower, allowing them to absolutely wreck the enemy with little danger to themselves. With aircraft carriers and a complenent of boats capable of coastal bombardement they don't stand a chance.

These two should already be enough to make the merfolk that are willing to attack humans extinct in short order, or force talks. They can have valuable resources or work power, which can be exploited in exchange for their lives/homes. Technology is scary and we fully exploited it during the second war. Repeating rifles, chemicals, devastating explosives at short and long range, timed fuses. Basically computerised warfare, rocketry and biological warfare are the only ones that still needed to be expanded.


What would admirals likely do in this situation, if the goal is to provide security for the vulnerable regions?

Naval officers, and indeed governments, would advocate the use of convoys - grouping civilian ships together, escorted by long-endurance warships, such as destroyers with small artillery and depth charges.

Considering your world is in the 1940-50s period, it's not unlikely to assume that your human navies might have weapons akind to the Hedgehog or Squid mortars that would throw a number of deadly mortar rounds or depth charges, killing any fish-person unfortunate enough to be in range of these weapons. It was also mentioned that the dwellings (kaiju-ships) were tortoise-like in structure and design:

...Like tortoises, these bioships have large bulletproof carapaces under which there are large cavities filled with air...

Depth charges and Squids (the weapons mentioned, not the six-legged sea creature) would both be deadly to these cities for the reason that the shockwaves caused by these weapons would crack open the carapace like eggs (since they sound similar to submarines.) It should be noted however that these devices would only have a maximum range of around 250 meters.

Is there some way to track kaiju-ships in the open ocean?

As has previously been mentioned by Starfish Prime, SONAR would be key to tracking these large moving masses throughout the seven seas. In addition to SONAR, spotter planes would be incredibly useful due to the fact that any large mass that isn't the same colour as the ocean is going to stick out like a sore thumb, making them easy pickings for aircraft with bombs, torpedos, or depth charges (smaller, but just as effective as ship-launched depth charges.) It should be noted that the kaiju-ships would only be able to be spotted by aircraft up to 100 feet, below this depth it would be quite difficult for aircraft to spot.

Or maybe it's better to reinforce static defences at the coastal cities, rather than trying to find a needle in a haystack?

Should your fish-folk decide to attack seaside resorts or cities, rest assured that they would be given the same warm greeting that the Axis gave the Allies on D-Day especially on Omaha Beach (but the attackers don't have guns to shoot back, so it would be a much easier time for the defending humans.)

The fish people would be attacked by everything the humans would have at hand including:

But wait there's more!

Since you noted that:

Alternatively, if the pillagers feel lucky, they might try to sneak upon the military ships in order to unleash acid fish on them.

Although this might be a novel idea (like as middle-age armies threw corpses into each other's castles in attempts to make each side ill), this would not work well. If the crew of a warship were under attack by a prospective fish-folk horde, they would need to either:

(A) Change the PH of the water around the warship, such as by dumping limes and lemons overboard - something which isn't unlikely considering these fruit could also be eaten by the crew to prevent scurvy. Another thing that could be done is to

(B) Electrifying the water around the ship, which would electricute the water, killing the fish and the attacking fish pillager people with the electric current in the water (the range would vary.)

(C) In the event that there was no time to perform the above two actions, sailors and crew could easily arm and throw grenades into the water around their ship(s). What would this accomplish? Although the explosions would cause minimal harm, the shockwaves that ripple through the water would be more deadly, potentially deafening, pushing away or causing other horrible injuries (broken bones, etc.) to your fish-folk pillagers (in a similar way to the depth charges already mentioned.)

  • $\begingroup$ (A) Changing the pH wouldn't work as it would be quickly dissolved (by the ship's propellers, even) - however, as has been noted, acid fish don't work too well in the first place, you need to inject into the target and that's pretty difficult for a metal hull. (B) Electrification is very short range - touch, essentially. (C) would work. $\endgroup$
    – toolforger
    Sep 15 '21 at 12:28

I think a more believable story for these mer-people not having much contact with 1940's and 50's America is that they had a treaty to stay out of each other's way and that trade would only happen through approved channels. All other contact was to be avoided, including having specific and approved shipping routes. Basically, we all knew about them and they knew about us, we just didn't mingle much.

After WWII, these mer-people either have a leader that's been wanting to forcefully change this treaty, or a coop/change in regime that wants to attack. They are under the assumption that the ammunition and equipment is completely depleted, and that the troops are so weary they won't fight. They would see a nearly constant stream of troops going into and out of the battlefield, so they would assume it's similar "attack and flee" tactics as they use.

What the mer-people don't understand is the pure manufacturing power WWII built up to be able to maintain weapons, ammunition, and other equipment nearly indefinitely. They also don't understand just how many people fought in WWII, which is 16 million or how any attack against the US would have initiated an immediate and extremely prejudiced attack. They also don't understand the fighting was fairly constant, rather than brief battles.

Essentially, the mer-folk would have borne the brunt of the anger of an entire nation just returning from a war, now interrupted in their celebrations and desperate to continue celebrating and getting on with life. This would be a short and brutal war with few mer-people survivors.


You mention "bullet-proof" shells of these bio-ships. I hate to break it to you, but that's not a realistic term. It might be bullet resistant for small arms, but not when you get to anti-tank or even anti-plane and anti-vehicle weapons. We're talking about .50 caliber weapons (at the smallest) that can go through 1/2" steel plating and cement walls. Anti-aircraft weaponry,

Going larger, you're talking about artillery, which even in WWII era is really big and powerful. The standard howitzer used shells 101mm to 105mm (roughly 4") in diameter and can penetrate hardened steel armor up to 7" thick or 18" of concrete. Your bio-shells might be able to handle a couple of those shots, but not many without significant structural damage.

And then when we start talking about dive bombers with torpedoes, we start talking about attacking the appendages and head of the bioship.


Americans had the Seawolf class submarine, which was supposed to be rated to 1600 ft, with a collapse depth of 2400 ft. Your bioships won't be able to avoid confrontation.

The torpedoes these submarines used, the Mark 14 were able to hit a max speed of 46 knots (85 km/h). These used 643 lbs of Torpex, which would be the equivalent of nearly 1000 lbs of TNT. That's a much more powerful explosion in water, due to it being incompressible and the pressure wave would be just as dangerous to a living being as fragments.

Navy ships

Iowa class US navy ships could hit 32 knots and ships from other countries could hit 45 knots. Your bioships aren't getting away from them, either.

Explosive fish

As another answer mentioned, these would have to be extremely powerful explosions and acid to do any significant damage to the equipment. Otherwise the acid would be washed off, the armored warships wouldn't even flinch, and nets would be erected around the decks to avoid the majority of it in the first place.


The mer-folk get obliterated in an extremely one-sided fight that ends up with them nearly exterminated and have extreme sanctions set up to prevent them from trying anything like that again. Their whereabouts would be highly tracked, the oceans would be continually heavily patrolled, and their way of life would likely end in a few decades due to the continual technological advancements on land they wouldn't have access to. Their younger generations would leave the bioships to join the land dwellers, even at the expense being subjugated and oppressed like the other minorities of the 60's and 70's civil rights movements would try to prevent.

In fact, the mer-folk would be another factor in the civil rights movements, adding to the pressure to repeal Jim Crow style laws and bring about more fair equality. Unfortunately, they would end up being a major focus of the opposition and suffer many casualties. But now I'm getting off topic.

Eventually, the mer-people would be accepted back into the international scene, but with the same sort of suspicion as any other country that's attacked the US in the last 100 years. They would hesitantly accept technology, but still be considered a 3rd world country, due to not producing any of it themselves. They wouldn't be impoverished/underfed/poor, but they also wouldn't be rich, unless they took over the majority of commercial fishing. If they were to do this, they would also be massively powerful and outspoken custodians of marine life and be working hard to reverse climate change on the international scene. But again, I'm getting off topic.

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    $\begingroup$ Not to mention those dive bombers and torpedo bombers will be a lot more effective if they are attacking target that isn't filling the air with AA fire/launching fighters. $\endgroup$
    – Rob
    Sep 15 '21 at 10:00

You know, everyone here thinks so hard about depth charges, torpedoes, naval artillery and other actual weapons, that they forget, that some military vessels of that time got the ultimate anti-merfolk weapon with them:

The active Sonar

Well, yes, you could use Sonar for tracking submarines and other big things below water.

But there is one thing few people take in consideration about sonar:

Sonar is loud

So what could your WWII-era surface humans do?

Give me a ping, Vasili. One ping only, please (hunt for red October)

-insert screams of merefolk here-

EDiT: You know, this could be your Plot Hook to make a first contact between merfolg and surface humans... up to that point in history, Sonar was supposed to make the Titanic no happening again, but suddenly there were pings in all oceans...

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    $\begingroup$ A good point. But a counterpoint is that these things are 4 times the size of a blue whale which can emit calls at 188 Db at the highest. Something 4 times the size might not only be louder but also capable of hearing louder sounds without any harm. $\endgroup$
    – SpencerG
    Sep 15 '21 at 15:00
  • $\begingroup$ @SpencerG That covers the kaiju. What about the inhabitants? $\endgroup$ Sep 15 '21 at 19:24
  • $\begingroup$ @LokiRagnarok I think that is why the biology of the Kaiju is essential. It would be likely that the interior of the animals where the merfolk lived would be impervious to outside sonar waves and harmless to the inhabitants. They are likely safe inside. $\endgroup$
    – SpencerG
    Sep 15 '21 at 19:53
  • $\begingroup$ Please dont forget, that decibel is a logarithmic scale with... a base of 10?. 188dB to 235dB in that case is a difference of 47dB, which is an increase of power of... 70.000 times? Just ballparking here... you will devaste any biological entities sense or hearing when it comes to close to that emitter. If they keep a some distance, they might be okay, but close quater battle against a Anti-Sub-Destroyer with directable emiters? Not a good idea. $\endgroup$ Sep 16 '21 at 10:29

Considering our WW2, there are potentially four areas of naval warfare to look at:

  • Battleships
  • Submarines
  • Landing craft and transports
  • Air power and carriers

Battleships were famously discovered to be obsolete early on in WW2, because they are very costly and are very vulnerable to both subs and bomber planes. Some even claim that Pearl Harbor ultimately helped the US by accelerating their transition away from a battleship-heavy navy. But if WW2 hadn't happened, presumably navies would continue to stay invested in battleships into the 40s and even 50s. So you have room for quite a few epic and bloody battles, up until the point where most of the battleships get sunk and the humans decide to try something different.

Submarines around this time would not work very well, I think. WW2 subs did not have much endurance, agility and couldn't dive well. The dolphin cavalry would easily locate the sub and kill it. Possibly the sub could get close enough to torpedo the bioship - but it would be a suicide attack. Later (nuclear) subs, like the Seawolf class, might have a chance if the merfolk cannot dive very deep, but otherwise are also quite vulnerable.

Passive ships, like freighters, transports and landing craft would be effectively curbed by the merfolk. The way you've designed it, they are perfect raiders. Presumably the bioships have infinite endurance, they can linger on sea lanes indefinitely, strike and flee. Invasions would also become difficult. Perhaps there's not much merfolk land to invade, but if it's a three way with two human nations, the merfolk could be a very powerful kingmaker between them.

The geography of your world comes into play as well. Countries on large landmasses, such as Russia, can easily move their industry inland and channel logistics into trains and planes. The merfolk will have little to do besides maybe sinking some fishermen and drive up sushi prices. But for island nations and archipelagos like Japan or Oceania, the merfolk can easily be an existential threat.

Of course I've saved the most interesting part for last: The way you've written this, your merfolk have no air defence whatsoever. They would be absolutely crushed by air attacks. Long range planes taking off from land bases (with no enemy planes to worry about, the bombers can be optimized for range at the cost of maneuverability), or carrier borne squadrons, will fly in, bomb the kaiju to hell, then the fighter pilots will get some live target practice with their machine guns and cannons - talk about shooting fish in a barrel. If the merfolk try to dive, no problem - the bombers can drop depth charges.

Survey planes can keep flying patrols and spot the enormous bioships easily on flat blue ocean. Underwater bioships at shallow or medium depth water can be spotted, same as anti-submarine aircraft do for subs. Planes can be equipped with radar or IR cameras. They can drop sonar buoys. The navy can build fake, "decoy" freighters to lure the raiding parties, and then kill them with the planes.

In our world, WW2 saw the advent of atomic bombs. During WW2 these were used on land, but after WW2 they were tested in the ocean, which was eventually stopped out of concern for the well-being of the fish and what not... Well, here you have the opposite problem.

Yet another element of WW2 was an interest in biological weapons. If your opponent is completely biological, developing some kind of contagious disease or poison seems like a very effective strategy. For example, engineering a virus that targets the bioships, or spraying the oceans with a poison from cargo planes, could be devastating to the merfolk. Worse, there would be fewer complaints about human rights violations, since they, well, aren't human. And also not covered by the Geneva convention? Uh-oh...

At some point you also mention 1950s. In our world, late 50s were a distinctly post-WW2 period, with nuclear submarines and carriers coming on the scene, as well as jets and very advanced electronics/sensors. A WW2 navy might obliterate these merfolk but it would at least break a sweat. If we're talking about something like the late 50s US navy fighting these guys, I think it devolves into something that is part funny, part sad. The environmentalist hippy anti-war protestors back home would be a bigger fight than the merfolk.


I think a WW2 navy would deal with these fish people with depth charges, torpedoes, and all the good anti-submarine weaponry you would expect. If we are taking the side of team humanity, the merfolk couldn't stand a chance, they would be forced into hiding. The coastal nations would reinforce their coastal towns and cities with normandy-level defenses, any fish people coming up onto land would be absolutely laid out.

Although I have a few ideas for how the merfolk could fight back. I know it isn't technically answering the question, I think that has been done quite a bit here, but it might be good to develop these merfolk to be a legitimate threat instead of an annoyance for us. So, the Kaiju. They could probably fight submarines. Seeing as they're huge, they're probably quite strong. If they could get within striking range they may be able to break submarines. From there, either the merfolk could storm the sinking submarine, slaughtering the crewmen, or just leave the sub be. Its fate is sealed, with damage of a giant sea creature fist right in its middle. As for how to get close, I think the best bet would be to wait at the sea floor if its at an approachable depth. Like near ports where they would have to dock perhaps... It would probably be impossible to differentiate it from the seafloor, according to my amateur's level knowledge of sonar in the 40s. That or they charge the submarine if they know that its alone. No risk of enemy reinforcement. At this time, subs aren't fast enough to outrun them, and if the kaiju had a good enough sonar ability, they could probably detect an approaching torpedo and evade. Although with a very close torpedo shot, they may be too big to avoid it. but once they get close enough its back to punching.

As for the suicide fish, yeah they cant do anything to the hull, but if there is enough of them, and their blasts strong enough, they may be able to damage more delicate components. They could damage propellers or rudders, or swim upwards to jump out of the water and on to the the deck, anything close enough to the water, maybe deck machine gun mounts, or depth charge throwers, could be damaged.

so like this, they wouldnt be incapable of damage, but it would still be quite an uphill battle.

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