I don't like it but as others have said, if you want a ride you have to sign. It doesn't absolve them if they are negligent... or maybe grossly negligent? I'm not sure, I've seen it explained in other threads by those that are in the know on legal boatie issues. Also, you can't sign away someone else's rights so that part of the contract is just there to scare someone away from hiring a lawyer. Once they hired a lawyer, he would explain it to them.
This is one of those things that is a result of scammers taking advantage of our legal system in the US. I'm all for suing someone if they really do something bad and injure/kill you. But if you do something dumb and injure/kill yourself the boat people shouldn't be hung out to dry.
So.. sign the waiver, or buy your own boat, or don't dive, or move to Venice Beach.