I am by far a doctor, but I've been in the optical field for 12 years, my suggestion would be to try a mono fit instead of the actual bi-focal contacts. My husband wears regular Acuvue lenses (he's nearsighted too) and has no troubles even at 100 feet. I can't see how the pressure would cause the Rx to change, seeing how contact lenses eliminate vertext distance (space between your eye and your Rx glasses). If it were me, I would call up th OD and ask to try the mono fit, certainly couldn't hurt....... hmmmm (ask him about depth perception with the mono fit and make sure he knows your diving)
Good luck