How to build watertight containers that keep water out even if it is fully submerged?
You need pelican cases. there is no need to reinvent the wheel here, you would spend much more money developing and building waterproof containers instead of buying readily available commercial products.
Pelican cases are used in media productions around the world to keep all kinds of camera gear, monitors and computers dry and safe.
I have worked on television productions that have used dozens of pelican cases to transport sensitive cameras and large media production gear to locations in wet and hostile environments here in Alaska.
I took my entire camera gear Outfit including multiple cameras and many lenses down the Grand canyon Colorado river In a 1620 pelican case, It kept it  safe and dry through all of the rapids  and being completely submerged for short periods of time. My laptop was in a pelican case in a raft that was overturned and submerged underwater for nearly an hour and stayed safe and dry.