One parallel is the practice 'eating the bitter'.
Another parallel is learning to control ones natural tendency to panic under stress. That is especially important while scuba diving. If one panics (for whatever reason) it could be fatal and there is the biggest parallel. It's deadly serious!
A scuba diver needs to be:
'Comfortable being uncomfortable'
An example:
Yesterday at the start of the second dive my mask was super foggy; I only had visibility in the upper right hand corner of my mask....That's it! So about 1/3 of the way through the dive I decided to flood my mask with sea water to clear the fog. It worked but then I had sea water in my eyes and it burned for about 10 minutes. Great for spiritual practice! You must be able to release into the discomfort, because you cannot just surface whenever your a bit uncomfortable because you're at depth and need to come up slowly and do a 3 minute safety stop before surfacing. There is NO shooting to the surface because you're uncomfortable. Self control is paramount to good scuba diving and martial arts alike.