Is it live or is it...wax? Actually, it's both. Baywatch star David Hasselhoff, shown with his doppelganger, was on hand for the July 26 debut of the Baywatch exhibit at the Hollywood Wax Museum, where its arrival incited anarchy. ''Fans climb over the rope and get right next to the figures,'' says a museum publicist. Explains a Baywatch spokeswoman: ''It's Baywatch, it's Gilligan's Island, it's The Brady Bunch. You know they're a cult phenomenon.''