Well, if you want to get into whys and wherefores, remember admiralty law has been around for hundreds of years and recreational diving, less than 50. Dive boats are barely a blip on the admiralty radar, as are most recreational vessels, for example jet skis.
The statutory prohibition on waivers is part of the tension among insurers, cruise lines and the public and came about because cruise ships were placing waivers of liability on the backs of their tickets. When the statute put a stop to that, they reduced the statute of limitation on claims to one year, which can really bite a land lawyer in the butt, by the way. Those limitations have been upheld.
In Schultz, the plaintiff tried to use the federal courts and admiralty law to escape the clear intent of the release signed by the decedent. The statute borrowed language from the definition of "coastwise trade" especially the "between" (different) u.s. ports and us/foreign ports. As I previously said, this distinction is why certain gaming "cruises to nowhere" escape our coastwise laws.
But as others have pointed out, I wonder what would happen if dive boat operators tried to use the releases to absolve liability for say, running over a diver with the boat.