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Here is the scenario:

Two factions, one defending its planet, the other attacking it. The winning condition for attackers is capture of leadership, so boot on the ground are required. The planet is Earth-like, the capital metropolis is coastal. WMDs on the surface are off the table.

Defenders have a wide range of equipment at their disposal, from surface-to-orbit missiles, to spaceships, satellites, planes, VTOLs, airships, choppers, tanks, boats and submarines.

Attackers are capable to launch and recover planes from orbit. They can launch troop transports from orbit as well. They have no lack of orbit-to-surface artillery either. They can match defenders in space, in the air and on the ground, but not really on the sea.

Attackers can rain death from above on anything on the surface, that includes boats. While boats can be used as a platform to launch aircrafts, troops and artillery strikes, those capabilities are redundant. Bringing in boats from space is somewhat of a logistical problem.

On the other hand, opening another front could have its advantages to divide defenders' attention. Launching planes/troops from the sea surface might also be less risky. It would be a new tactic, so there is the element of surprise to consider.

The first question would be: Is it even worth taking the fight to the sea?

Now, assuming we want to fight aquatically, attackers have developed a new type of vehicle, dubbed Space/Naval Hybrid Assault Platform (military loves acronyms). It is a space-worthy ship specifically designed for the task of landing on water and floating thereafter. It cannot takeoff without assistance, and cannot fly in atmosphere beyond controlling its descent.

It could conceivably be single-use, e.g. you could disassemble it for building materials for a base of operations on land, or you could just beach it for similar results, or it may just be a fancier carrier/assault boat.

So the other question would be: What are benefits and drawbacks of such ships, and what would they best be used for?

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    $\begingroup$ the curved nature of the surface of our planet means anything floating on the surface is less easy to spot than anything floating in the air or in orbit, simply because there's a planet in the way. So detection would mean either air-or spaceborne onlookers, or close proximity. If you go one step further (or even, deeper) create space-dropped submarines, for added stealth... $\endgroup$ – Burki Jun 9 '17 at 10:08
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    $\begingroup$ "Is it even worth taking the fight to the sea?" Yes, in a way. The d-day invasion came to my mind while reading your post. If your story is a reference to that (or something similar), it is worth going for it - do not worry if this would actually happen (planetary invasion will most likely never happen), worry about selling it to your audience. Btw, an aircraft carrier on the ocean would still be a way to save fuel if this is an issue. It is very fuel expensive for fighters to enter space every time after a mission - and it would be very difficult when damaged. $\endgroup$ – Raditz_35 Jun 9 '17 at 10:33
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    $\begingroup$ @JDługosz it does make a difference. the observable region is smaller, or to observe a region requires a bigger investment. with surface based radar or similar you should be able to track spacecraft in something like 40% of all possible places they can be, but a lot less than 1% of all places on sea. $\endgroup$ – Burki Jun 9 '17 at 10:51
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    $\begingroup$ @MarshallTigerus To get them to surrender. $\endgroup$ – AmiralPatate Jun 9 '17 at 17:35
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    $\begingroup$ @Pak A) Attackers aren't interested in annihilation, making such a threat toothless. B) There are treaties with ways to enforce punishment, so that's way off the table anyways. $\endgroup$ – AmiralPatate Jun 11 '17 at 14:40
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Sea warfare is kind of deprecated already.

The only real reason major global powers still have a navy is because, compared to a ship, planes need to refuel and re-arm often. If you have very fast space-borne/airborne vehicles, that don't need to re-arm (energy weapons) or can do so quickly (high-speed propulsion) and need to re-fuel only once in a while (fusion-reactor), what use could a ship be? Such a vehicle would be way superior to any seaborne vehicle. You can see such a trend already, all major powers have/want aircraft carriers, and their only weakness is being a ship (vulnerable to submarines). BTW, why do you even have planes when you have space-ships (no atmospheric-flight capable engine?)?

Considering the advanced technology level, I think it's unreasonable to think that your attacking force shouldn't be able to field a constant military presence going from orbit to sea-level above the defender's capital city. All this without ships.

Even today, planes very rarely engage in dogfights. Using missiles, engagements are performed at many miles distance. Why shouldn't your orbital artillery, space ships & orbital-deployed planes be enough to suppress any naval fleet? You don't need a ship to destroy a ship, you just need a weapon that can reach it. It isn't that easy for a ship to hide, since the sea itself doesn't provide cover from sensor detection, more so if the sensor scans from orbit. There already are anti-ship missiles systems with ranges of almost 1000Km, space is just a few hundred Km away. Why would you deploy ships when you just can launch swarms of missiles from orbit to kill any seaborne vehicle? You could even just drop space debris or some metal pole from orbit onto the ship, ships are slow, and the impact of such an object near the ship would have the same effect of a nuke.

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    $\begingroup$ why do you even have planes when you have space-ships Aircrafts have to be aerodynamic, spacecrafts don't. Making all planes space-worthy would have unnecessary costs. $\endgroup$ – AmiralPatate Jun 9 '17 at 15:47
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    $\begingroup$ And the other way around, it's much easier to make spaceships that don't need to be able to enter an atmosphere. When they do your engines must be capable of lift off under full gravity. $\endgroup$ – Mormacil Jun 9 '17 at 19:51
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    $\begingroup$ @AmiralPatate, developing, building, maintaining & deploying a separate fleet of vessels is a logistic nightmare compared to having a space-fighter capable of operating in an atmosphere. You don't need all your space-ships to be capable of atmospheric-flight, just one particular class. There are already many development projects of engines capable of atmospheric and space flight, to believe such an advanced civilization wouldn't choose the same approach is just nonsense IMO. You will need to enter the atmosphere with a craft to take a foothold on the surface sooner or later anyway. $\endgroup$ – r41n Jun 26 '17 at 6:39
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It might be worthwhile from a couple of perspectives.

The advantage you are looking to secure is a speed advantage. Launch an attack from 3 miles out and your defenders will have a lot less time to respond. Attack from orbit, there is actually more distance to cover, more time to prepare, and so on. Outside of lasers in space, projectiles of any sort have travel time, and that time can be used to defend. Hyperspeed type projectiles are too devastating to be used willy nilly around population centers according to your rules.

You can use your SNHAP to get to the surface in areas where the defenses are thinner. Have lots of troops and armaments on board.

Don't land on the surface, go underneath. You will be harder (but not impossible) to detect. Land your boats all over the oceans, and have most of them go dark and move around a lot. Have one hit a coastal target on one side of the planet, hit another on the other side. Keep up some distracting stuff for a little while.

Mass the troop carriers and logistics boats for an amphibious landing assault on the capital city. use last second concentrated aerial attacks to get the defenders looking up. End the aerial bombardment with mass fire on the coastal defenses, then start the ground assault.

Why do this? You could gain a pretty good logistical advantage. You could get your troops to the surface in relative safety and move them to where they are needed, along with the support equipment and so on. If you lose a supply boat, no problem, send another because the main assault is a while away. You get to use the SNHAP to distract the defenders and even demoralize them.

There are psychological boundaries your defenders will have and you are breaking those too. The defenders need those oceans, and so will hesitate to take drastic measures. Think about how hard it can be to work yourself up to go see a doctor to have some sort of surgery. It's your body, it will hurt, even though the whole thing will be ultimately beneficial, you still don't want to do it. That will translate to "If I destroy these U-boats, it will cause a lot of damage to the coastal ecosystem" Time runs out while they try to both protect the coastal ecosystem and find ways to destroy the boats.

I don't necessarily think that these SNHAP's will be a total game changer, but it's not a useless idea by any means.

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I assume that the leadership that needs be captured resides in the coastal metropolis. The attackers are practically 'laying siege' to the capital, with the objective of breaking through the defensive perimeter and occupying it, capturing the government and making them capitulate to their terms.
Initially, I would go with a Special Forces operation to achieve that objective, instead of planet-wide warfare. But that defies the assumptions of the problem.

The sea, for this coastal metropolis is
(a) avenue of resupply/logistics support and
(b) open escape route.
For both (a) and (b), the attacker (whether they like it or not) will have to take the fight to the sea.

To achieve the above objective, as it seems that the capabilities are more or less balanced, the invaders would have to control the naval theatre of operations. However, considering their space position and advantage, I would say that it would not be worth it to build naval capabilities.
Instead, invest the same resources into obtaining space and air superiority. This will achieve the same result even over the seas - the defender's navy will be slaughtered without air support.

So, to the second question, my answer is: such ships would not be cost-effective considering their capabilities.
Any capability they have (troop carrier, air carrier, etc) and, most importantly, the effect of carrying out the capability (troops on the ground, air superiority, etc) can be achieved by other means, with existing capacities in greater number.

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  • $\begingroup$ Air superiority can be tricky against a navy without air support, but efficient anti-aircraft defenses. A combination of SAM with CIWS against incoming bombs can make air attacks against the fleet very risky. Orbit bombing against a moving target is going to be tricky, due to the time the projectiles take to hit the target. $\endgroup$ – Rekesoft Jun 9 '17 at 10:49
  • $\begingroup$ @Rekesoft How having actual ships on the sea is going to change this? $\endgroup$ – adonies Jun 9 '17 at 10:54
  • $\begingroup$ In nothing. While sci-fi has a trope of future battles being decided by who controls the upper orbit, actually I think it's like sea blockades in the age of sail. The blocking fleet has to be resupplied at great cost - and usually, with quite long delays between asking and receiving - while the people being block have much more resources. I agree with you it's not worth to build naval capabilities, but aquiring air superiority is probably off limits, too. $\endgroup$ – Rekesoft Jun 9 '17 at 11:28
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Ships would be obsolete.

Boats can't adequately protect themselves from orbital bombardment, and same goes for cities. If you want to overcome this problem, introduce the concept of "shields", so, like in "Empire Strikes Back", an attack can not be launched directly from space, but rather via ground (or sea) based force fighting close combat.

Without meaningful protection (like from using shields), losing orbital superiority would be an endgame for the defenders. They can still burrow underground and launch counterattacks, but from that point on they will be the mice in cat and mouse game.

What if an attacker can't establish orbital superiority? Then defenders can stand for quite a long time, although attacks on them would be exhausting, like German air raids on London during WWII. For attackers, it would not be practical to bring the fight into the sea, because this is where they are the weakest. It would make much more strategic sense to try to establish a foothold on defender's planet using their strongest assets.

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