I never assume a release or waiver will or won't be effective. I will always assume that a court will do whatever is most injurious to my interests. Subject to this, in California, it is possible to waive claims for basic negligence and absent some mistake like labeling the waiver something it is not, it is likely to be upheld by a court.
Apart from waivers, despite their reputation for being "pro-victim," California courts are pretty aggressive about applying the doctrine of primary assumption of the risk. In a nutshell, it says that if you are injured as the result of a risk inherent in the particular activity, your out of luck.
Being hit while playing touch football is a risk inherent in the game and players can't be liable unless they intentionally tackle another player.
The California Supreme Court has held that in golf, “being struck by a carelessly hit ball is an inherent risk of the sport.” However, it also held that in the case before it, there was a question whether the defendant’s conduct was “so reckless as to be totally outside the range of the ordinary activity involved in [golf]."
Getting hit by the swinging boom of a sailboat is a risk inherent in sailing a boat for which the skipper or owner cannot ordinarily be liable.
While snow skiing, colliding with another skier is an inherent risk of the sport and there is no liability unless there is reckless or intentional conduct. However, while racing down an advanced run is part of the thrill of snowboarding, intentionally speeding into a flat area at the base of an advanced run where people have stopped to rest, when one is unfamiliar with the area, without looking where one is going is not necessarily an integral and unavoidable part of the sport.
I do not recall any California cases dealing with primary assumption of the risk and scuba diving. However, I would hazard to guess that taking an "undeserved" hit is an inherent risk, as is getting tangled in kelp, or stung by a jellyfish or getting poked by an urchin. So, too, is OOA. I would also hazard to guess that getting "bad" air in a fill is not an inherent risk of the sport. Nor is getting left behind. Nor even getting defective equipment. I'm not sure about getting a "short" fill, especially because it is generally so easy to ascertain and address. (It might trigger any number of other doctrines.)
BTW: I would not have suggested the caveat if I was positive that a waiver would not be upheld in a left at sea scenario.