How to waterproof electronics for an underwater project?
15-20 ft underwater is a about 7 - 10 psi (0.5 - 0.7 atm). A sealed polycarbonate box thick enough to withstand that pressure should do the trick. If you need cables to/from the electronics that’s a bit more work.

You can also consider filling the enclosure with a nonconducting, incompressible fluid such as oil. That’s what they do with stuff that has to go really deep (think ROVs like they use on oil rigs.) Of course your components cannot have any air voids, and they have to be compatible with the fluid.