How would underwater weaponry and technology look under high pressure?

Question

Suppose we have a fictional water planet. Humans have discovered interstellar travel and have colonized the planet. They live in large underwater cities. Its oceans are under high pressure, say perhaps ranging from 1000 psi to pressure similar or even higher than at the bottom of the Mariana trench (some 16,000 psi). There are hostile animals on this planet that are pretty effective at attacking submarines and humans and are durable. Humans would have advanced technology and weaponry such as practical railguns and coilguns.

So my question is, how would weaponry and submarine technology look like on this planet?

Further Elaboration: Would the submarines be similar to today's deeps sea submarines? And for the weaponry side of the question, would it be feasible for people to be in suits at lower depths? And would handheld underwater firearms be even feasible, or would humans be staying inside their cities and submarines all the time? If underwater firearms and such are practical, how would they look or function at such high pressures?

Extra information on the world in question in case it matters:

The planet's surface is 78% water, it has two moons which create kilometer-high waves and turbulent seas. Land on the surface consists of a few small continents and thousands of islands ranging in size from a few hundred kilometers to a kilometer or two. The islands and continents on the surface are also barren, being little more than resource-empty rocks, and humans decided that it wasn't worth settling on any of the islands. The surface is practically inhospitable due to these conditions, and is all but uninhabited except for a station for spacecraft to land and take off. However, under the water, there are tunnels and large caverns, results of ancient lava flows. The planet also has a lot of radioactive ores that decay and produce most of the heat deep underwater. A result of this is some areas of the ocean are boiling and some are freezing.

The planet's inhabitants would range greatly in size from small to larger than the largest submarines. They're also hostile due to aliens (humans) entering their territory, so that would be their reason to attack. Many of them can tear through the hulls of submarines, be it through ramming, tearing, or piercing. They are also naturally very robust, due to their harsh environment. As a result of hostile fauna, submarines and their crews would be armed. Humans came to this planet with the intention of harvesting its resources. The surface station harvests gasses such as argon and nitrogen. Its cities center around mining ores, rare earth elements, and other valuable resources. Submarines function to travel from city to city, remote station to remote station, transporting goods among other things. [In this universe] this is the first planet humans have encountered extraterrestrial life on, and thus there are also missions to hunt or capture creatures for research. and thus, they are motivated to build stations and cities at the lower depths, wherever they are after resides.

Response to questions:

The ecosystem's nutrient base would come from a combination of geothermal venting as well as heat created by the decay of radioactive ores and metals. The surface of the world isn't exactly uninhabitable per se, humans just aren't residing on the surface except for one installation that connects the planet to the rest of civilization because it's simply not worth it to settle on the surface. There's barely any resources and the surface is plagued by constant violent waves making it dangerous. The surface oceans are also rather barren, most life being located within the planet-wide system of tunnels and caverns. Humans settle down about a kilometer or more into the tunnels or wherever the resources are.

Also, I am asking this question under the presumption that humans would and have settled on the planet and are making technology for use on this planet. ANd humans are focusing their efforts on settling in the cave systems because the surface has a violent sea that has constant multi-kilometer waves thrashing settlements. The atmosphere is also only 13.5% oxygen.

Please note that I am asking what would such tech look like under these conditions, not the practicality of humans settling on such a planet.

Answer 1

If I were colonizing such a world, with a strong motivation to settle there (unobtainium, lack of better real estate, whatever) I would have population centers either on the coastlines, aboard giant ships, or in underwater neutral-buoyancy cities high enough up to allow easy access to the surface. Ultra-deep enemies probably couldn't tolerate LOW pressures, so humans would have safe harbors so to speak. People would bring earth like conditions with them.

• Submarines would be jet-powered improved versions of the ones we have now, but weaponry would be primarily robotic/drone based, with few people operating large numbers of mini attack subs. Organisms in water would be very vulnerable to things like depth charges and exploding torpedos. Gravity would favor humans raining death from above.
• Open ocean would be easier to control, but extensive caves (I'd put unobtainium here) would be challenging as control of drones would be hampered and distances shortened. You can't use big explosives because you don't want to destroy the caves! I'd move people to sealed bunkers to keep them safe. Coming and going from mines would be danger points.
• Drones would use torpedos, mines and depth charges, possibly poison harpoons. Sonic weapons would be interesting; The super-whales of Earth are thought to have used sonic attacks in their hunting and may have been able to kill with them. Look to terrestrial sea life for drone designs; Octopi, puffer fish, electric eels - the possibilities are endless. There was a military sci-fi series set in the ocean, Seaquest. If you recall it, That would be good source material as well.

Answer 2

They wouldn't settle on the planet at all.

If you've got the tech to build a working underwater habitat, you've also got the tech to build working space habitats - and while the ambient environment is no less deadly, there aren't angry sea monsters trying to kill you in space. If they really felt the need to settle on any stellar body at all, rather than just setting up shop in asteroids and artificial space habitats, they'd probably decide to settle on the moons, instead. Any resources that they could conceivably get from settling in the ocean can be gained from asteroid mining.