For me, waterparks dont need to be ridiculously-themed or extravagant. Waterparks dont typically give off the controlled environment vibe a super-themed park needs to pull off. People are walking around half-naked and covered in sun screen and sweat and water...sure, some theming is nice but nothing too crazy. And I feel like most guests would feel the same way.
IMO, Id rather keep Wet N Wild which is cheap, has low crowds, and focuses more on slides. I know that will be an unpopular opinion on here but Id be more than happy if Universal simply made cosmetic changes to WnW rather than level it completely and build a destination water park that will be expensive, crowded, and become an all-day ordeal to get done. Waterparks are best as afternoon visits rather than all-day affairs a la theme parks. Just my 2 cents.