Lazy rivers act a bit strange, they are kind of an attraction in an attraction. It is best that you probably cut lazy rivers from actual pools. So your mistake was to build a pool entrance, just build a lazy river without the actual entrance, seems to have the biggest sucess, if you build the entrance the guest think more it is a pool and want to do 'pool things' and not go on the lazy river.
Edit: After playing with it, i got the idea, what is wrong: The game does not seem to calculate Pool depth properly, but guests do: So you have your pool: with diffrent depths the game take the minimal pool depth. (wading for example) and calculates the pool capacity. But guests seem to have a tolerance for the depth of water. This causes the issue with the mixed areas.
Example: you lay out 4m straight line pools: Wading - Swimming - Swimming - Swimming - Swimming. The game may calculate that 20 people can go in the pool, if the 20 people all have tolerances for wading, the people will flock around that piece and kinda chill on that square. People in wading water have an animation bug.
So to avoid this issue, either to a gradual pool with lazy river exits, the pool should be the same size or similar size of the river (roughly) or you do the single lazy river, all swimming height.
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For the swimming thing: The River doesn't have directions (of course there is current, but not strong enough) and it is still a pool. So if a guest had enough 'pooling' he will get to the nearest exit, if this means swimming against the current.
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Just some hint on troubleshooting in general (not directed at anyone): Check your guests, like literally click on them: What is their current activity? Are they a group/family: if so, what are the others doing, this is tedious: try finding guests with the same last name. Especially in pools, the guest ai is mindbaffling in a good way: I had serveral Why? situations, where at the end i discovered: yes, it makes a lot of sense actually. For example mum and dad waiting in the pool while the child is on the flume, which led to the pool.