Sounds like an interesting project! Have you built things like this before? If you have any links to similar projects, I'd love to have look!
How deep will the sub go? If it's not going too deep, how about rebuilding some equipment for water cooling and running the pipes through the hull, that way you could use the water temp from the outside to cool your elements? (Via ribs/radiators, not recommending you to pump salt water into the cooling system)
Or, if you need the casing to be more tight, use metal ribs like the ones on/in old hi-fi amplifiers, and make a metal connection from the cpu/chipset to the ribs? If you find big ribs, you can make them run in parallel with the hull, so they won't affect speed, but might help stability?