The park is built and there's really no space for anything like this here anymore, but in my mind, a water theme park should just be a largely traditional theme park where you can get on a water ride and not have to worry about getting soaked. Does that make sense?
Everytime I'm in IoA, or DAK, or SWO, and walk past any of their flume or raft attractions, I will hard pass most of the time. Because I'm in shorts and I'm wearing socks, and I've got the whole day ahead of me, and I really don't want to get soaked on Popeye's right before waiting in that indoor Kong queue haha! In a water theme park, I should walk into whatever Dudley-style ride they've got willingly, because I'm already prepped with swim trunks, sandals, sunscreen, and because getting soaked is the whole point! Slides and things like that would be fun supplementary attractions, but I want actual full-potential water rides that I don't have to stress about ruining my day!
It's all semantics at the end of the day, but as wonderful as Volcano Bay is, it's really just an improved version of something like Blizzard Beach. Not a "water theme park", but a "themed water park".