The actual pressure difference it quite low. One atmosphere (about 14.5 PSI) is about 33 feet of water. If your prop shaft is 3.3 feet under water the seal only has to withstand 1.45 PSI pressure difference. This is a point so many people don't realize when the talk about seacocks. Your faucets need to handle far more pressure.

The point I am trying to make is you don't need to put 100 psi pressure on it. It may hold 100 psi but it might just fracture. If you can plug both ends of the tube, pump it up to 10 psi and it holds for an hour you are done.