I think corrosion is the big issue here. perhaps that is where the leak comes from.

Water under high pressure will find any weakness. (For example, the places where the wires go in and out.)

Corrosion indeed is always a problem.

Not if the datacenter itself is operating at the same internal pressure as the water around it

Interesting point. But also consider:

1. pressure may compromise the electronics

2. water pressure increases very rapidly with depth. You'd have to pressurize the data center with a liquid with the same density

3. even a small differential will produce leaks

Air (or rather, nitrogen -- oxygen at that pressure is something you'd rather avoid) can be pressurised to that degree too: https://en.wikipedia.org/wiki/Moon_pool