My theory follows simple logic.
The peep groups are attached to, and controlled by a "flow particle". Animations of guests revolve around this particle.
This is why peeps can't be split up. Decision making is per particle. This why we can have 10,000 peeps.
In RCT3, peeps were controlled individually and we couldn't have 10,000 peeps. In PlanCo, 10,000 peeps may only be (on average) 3,000 flow particles to control.
Since the peep groups can't be split, single rider water slides are out. Or, they would have to load up to 6 peeps in the flume per "ride" and you know people would complain about that mechanic being unrealistic, just like they do for the coaster, and the track ride that releases 3 cars at a time, essentially treating them as a train.
Pools are out of the question too. In RCT3 - Soaked, when peeps went into pools, the family groups were free to split up and use lounge chairs, hot tubs, diving boards, and slides individually. That is impossible with PlanCo's flow particle crowd simulation. Suggesting they change that? It won't happen. It's a baked in limitation.
We may see more water rides like the log flume or river rapids, that have "cars" that can hold at least four peeps.
I think flat water lake rides may be problematic too. The swan boats, jet skis, canoes, etc, had no defining "ride track" and peeps were making their own decision on a per boat level, that also reacted to a user defined "terrain". If water flat rides do come, I'm willing to bet the operational area would be predefined. Which everyone will complain about the lack of customization. Guaranteed.