Whenever my family went on holiday to Dartmoor when I was little, parents would always take us to church at Buckfast Abbey if we were there on a Sunday.
Always thought it was just for the nice architecture and the gardens, but maybe they had another motive....
Would depend on how deep underwater and the pressure and so forth, but maybe look at how steam engine piston rods are sealed - an o-ring or a wiper seal would probably do the job