There are a number of conformal coatings that can be used to keep a circuit board dry at fish tank pressures. Parylene is my favorite as it isn't too harsh on the board materials and coats even underneath the surface mount components. Doesn't usually stick to the board though so it's more of a perfectly sealed bag around the board. It's good for basically forever.
If you unplug the connectors you rip a hole in the "bag" though and the connectors will corrode. The way around that is to use expensive hermetically sealed connectors that are designed to operate underwater. Each side of those will cost a few hundred USD for boring round connectors. I have never tried to run e.g. HDMI through a bulkhead but such a connector would probably be impressively expensive.
Anyway the best thing about Parylene is you have to send your equipment out to have it applied, no do it yourself here. The spray on coatings are usually sticky, messy, and not that great overall in comparison.