Mostly covers for different hoses and valves for scientific instruments used underwater. For the seals I mostly use O-rings.

I myself have made a couple of very simple watertight (non essential) prints for scubadiving. They are holding up really well!

The guys from Prusa Research have a lenghty blog post about watertight 3d-prints: https://blog.prusa3d.com/watertight-3d-printing-part-2_53638/ This might be a good source for tipps and tricks!