Ten years ago RCT3P had better water than PlanCo does currently, and was able to resolve all the issues you've brought up, on Frontier's in-house game engine which they have 100% control over. Variations on "but it's too hard, it's unreasonable to expect that!" are not persuasive given the history of the genre.
In a RCT3P park I was still fiddling with just before PlanCo came out I had an Atlantis area which could only be reached by underwater paths -- can't do that in PlanCo yet. The paths on the beach had water cannons -- can't do that in PlanCo yet. I'm not particularly interested in waterparks or putting pools in my parks (although they'd be nice beside the hotels), but I very much want what a water expansion should include. I'm willing to pay for it. Judging by the positive responses to the OP, I'm not the only one.
Oh, I don't disagree with you, RCT3 has MUCH better water and features connected with water. But PlanCo isn't RCT3. Even if they share a version of the same game engine, they aren't the same game, they aren't designed the same, they don't have the exact same coding, they don't have the same resource requirements.
My point is that we don't know why Frontier decided to go with this type of water for PlanCo, was it perhaps done to limit use of resources that they wanted to use elsewhere in the game? Would changing to RCT3 style water cause problems for PlanCo as it was built? Is it because changing to RCT3-style water isn't hard, but the work needed to tweak AI and everything else to use pools, etc. a problem? I certainly don't know, and I doubt anyone else here knows for sure either.
I just think that Frontier likely has reasons why behind these decisions, behind the reasons for this water design and behind the reasons why they don't plan to alter it. And they may well be very good reasons, we simply don't know. Stating that they were able to do it in a game ten years ago still doesn't address the very real possibility that there maybe are plausible reasons why they didn't do it now. And I think some people (not referring to you) perhaps don't take into consideration that there might very well be specific reasons why water parks won't, or can't, be added, no matter how much they would like them, and no matter how frustrated they may be that they don't get them.
Some other people have brought up hotels elsewhere, and how they did get them after being requested quite often, but they don't like how they work. Perhaps there's a very good reason why Frontier designed them that way. Perhaps it was just a small addition that didn't really put a hit on game resources like the addition of pools and peeps changing clothes and swimming would, or like more dynamic water would. I don't know myself, but I think that's a realistic possibility. They shouldn't compare the simple addition of adding hotels to what I imagine is a far more complex process needed to add better water and water features like pools.
IF Frontier changes their mind and decides that they can tweak the game to offer water parks, I'm all for it and would
gladly pay for such an addition. I'd still like to see other additions to the game as well, preferably before a water park, but Soaked! added a lot to RCT3 besides just pools and peeps in bathing suits.