A cheap stereo microscope will cost about $100 and will make surface mount soldering much easier.
The following teardown video will give you an idea about what goes into building a safety critical waterproof pressure sensor system.
It isn't for underwater use, but the principle is the same.
At 20 bar anything with air in it will be crushed. This includes electrolytic capacitors, voids in components etc...
Truly waterproof pressure resistant containers aren't cheap. If it leaks will the electronics survive?
Do any of components behave differently at elevated pressures? The manufacturers might not be able to provide that kind of info unless you look for aerospace grade components.