Creating impermeable pressure housings with FDM printers is difficult. Strength is not the issue here, a thick housing in any material will be plenty strong. Dealing with permeability from gaps between the layer lines is the challenge. ABS, PET/PETG, polycarbonate would work. I wouldn't do this in nylon or PLA. To have any shot at low permiability you should print HOT and SLOW with a good bit of increased extrusion and line overlap. 100% infill obviously. Use dry filament. Reduce cooling fan. Play with settings and look into the techniques used by people trying to achieve very strong layer adhesion or optically clear parts.
If using ABS you could vapor smooth it with acetone to create a fused surface but even that isn't really impermeable.
Rigorously test anything you make before putting a camera in it.
Resin printers might work well for this but I'm not too experienced with those.
The "salt remelting technique" is a very viable option, or to go another route so is using lost-PLA casting to make the parts from aluminum based on a 3D print.
Acrylic is marginally more scratch resistant than polycarbonate which turns out to be the biggest reason it's preferred for this sort of thing. Laser cutting and solvent welding an acrylic housing is a very reasonable option.
For the buttons, I'd basically copy what almost all camera housings use. A shaft with a spring, o-ring, and a couple of circlips. The walls of the hole will have to be nicely drilled or polished for the o-ring. Using a machined metal sleeve glued into the part is an option but I'm not sure if that's better or worse in this case. You can probably find something like that off-the-shelf.
I've been meaning to write up a more detailed post about 3D printing stuff for underwater use. At some point I'll get around to it if enough people are interested. In the meantime feel free to hit me with any questions.
