I've seen this sort of debate on the boards many times. After my three year hiatus from the board, it's depressing to see that people still rely upon the myth that waivers are not enforceable. For those who don't know my screen name, I don't play a lawyer on television--I'm the real thing.
SOME waivers are not effective. That is because the waivers do not meet the requirements for validity or enforceability. When a waiver relating to a voluntary activity is not enforceable, it's usually because the lawyer who drafted it screwed up (yes, it happens) or the operator failed to insist upon the diver reading the waiver and executing it.
In the case of a dive off the coast of the United States, Federal Maritime law governs an accident if the plaintiff so pleads. Also, Federal law will govern the efficacy of a waiver when the accident occurs within its territories (e.g. Puerto Rico, USVI). Therefore, the waiver must meet the tests for validity set forth under maritime law. Those requirements are fairly straightforward: (1) whether person signing waiver had informed consent; (2) whether clause was inconsistent with public policy; and (3) whether clause constitutes valid adhesion contract. Incidentally, state law (at least in the states where I'm admitted to the bar) is identical.
The first prong relates to whether the waiver specifically states what rights the diver waives and the potential risks the diver is assuming. The second prong measures whether a release under the circumstances somehow harms the public interest. Courts have repeatedly stated that no important public interest is harmed by an individual waiving rights when that person voluntarily elects to participate in a hazardous recreational activity. The final prong seeks to determine whether there is inherent unfairness in requiring the waiver or a lack of bargaining power between the diver and the operator. If the waiver meets all of these requirements, it is valid.
In the most recently reported Federal cases on the issue of waivers in scuba diving, the waivers have been upheld as entirely valid and enforceable when the waivers meet the above criteria. See e.g. Murley ex rel. Estate of Murley v. Deep Explorers, Inc., 281 F.Supp. 580 (E.D.N.Y. 2003); Olivelli v. Sappo Corp., Inc., 225 F.Supp. 109 (D. Puerto Rico 2002). However, the other prediction of Murley and Olivelli is that waivers that merely say "I release the local dive shop from liability for all injuries" will certainly not be enforced (prong 1, lack of informed consent). Therefore, careful drafting is crucial.
State law will govern accidents occurring within the confines of state borders, so if you're diving in a New York lake, you should look to New York law (which, incidentally, holds that scuba diving waivers are entirely valid).
I hope this clears up confusion.