What if the mer people only lived at great depth? The supermarine would need to be able to contain highly pressurized water, and require pretty hefty construction in that case.

@Innovine Which is why I posed a question concerning just this to OP in a comment above. As you can see they responded that merpeople live up to the surface of the water. So it's fine.

Hello, please take my upvote for correct naming of device

Keeping the water oxygenated would definitely be a problem. An easily solved one, but still something needing solving, and equipment (bubble machine or something) which can break.

Also, keeping the supermarine from leaking would still be a problem. In a submarine, you can always pump any water out, but in supermarine you wouldn't have "air leaking in", you would still have water leaking out, and no way to get the it back. Now I'm not saying this is a hard problem, but it still needs solving, and can't be just dismissed as trivial.

You got a down vote for me because you're not taking the question seriously, and in fact mocking it, There is enough of world built where answers from glass fish tanks to metal structures can be used, yet you ask for more "entertaining responses"

"In fact any technology they'd have would already be build to be able to resist said enormous pressures" – Why? If you build a machine or vehicle water on the outside and water on the inside, it's not going to have to withstand anything special, because the pressure is the same inside and out.

@TannerSwett And this is why I don't pride myself of being intelligent. Because I'm not. Good catch.

What if those merfolks need to keep the water highly pressured?... like if they were used to live 300m-1000m underwater

@PaperBirdMaster Unless the merfolk was microscopic, that's not possible. There's no material which could hold a few cubic meters of water at such a pressure against 1 atmosphere of outside pressure. Any known material would expand to lower the pressure, or just burst. Only gravity (ie. actually being hundreds of meters underwater) can do it.

@hyde Oh... I was dreaming about a tank-like metal box with wheels filled with merfolks. :'(

Yeah @hyde - good point, I'm still trying to problem-solve this, too. Maybe that is a good follow up question once this is nailed down.

You should throw in some fag packet calcs about how much more a tank or an APC would weigh if you filled the crew compartment with water. My guess is a lot.

@GeorgeMcGinn I tightly grip my right to mock literally anything and everything I wish to. As long as it's not abusive mockery. Which in this case, since it was just a light jest, it certainly wasn't. In addition I didn't ask for more entertaining responses, I was quite sincerely concerened OP was expecting something I little more substantial: it can be disappointing to realize the solution to one's problem is banal.

Consider the fact that it's going to weigh about 3 tonnes per crew member just for the water they'll need, "light" is probably not the word.

@PavelJanicek, a supermarine would move above the water, this moves above the land, perhaps a superterra.

@hyde Ocean pressure at 1000m is about 5000 psi. We regularly build pressure vessels to withstand about twice that amount to store liquid hydrogen, so it's certainly possible to do so...

Here's an actual picture of a supermarine

@ckersch Hmm, cool. I bet those tanks can't have windows though... Of course that is "easily" overcome by cameras etc.

@hyde Or something like a periscope. Submarines are window-free for similar reasons.

Because the water is not as transparent as air. Windows are of little use at the ranges at which submarine combat happens.

Which also raised my question "How do the mermen see?" They might use some sort of biological sonar, which would not work in air.

Or they have optical vision, which is rather short-ranged to what we land-dwellers are used to, even though it's sufficient for close-range combat. While a long-range sniper gun would be technically possible under water (the projectile would have a tip which causes supercavitation and therefore be able to "fly" through the gas bubble/channel it is creating, see en.wikipedia.org/wiki/Supercavitation - they even mention that supersonic submarines are reportedly in development...).

Well, assuming that mer people are evolutionary developed from fishes, we could assume some sort of optical vision. Of course, one might also assert that they are descendants of lungfish...in which case limited air breathing abilities would be present.