I hope this message finds you well, present circumstances excluded. As a matter of course, he could be evicted based on a breach of the contract
(assuming the rental contract prohibits activities such as this). In addressing the issue and giving him legal notice while also protecting yourself, it would probably be best to give him a written warning via letter that his activities in the pool and elsewhere are a violation of his rental agreement. Tell him if he is caught again, you will be forced to evict based on his breach.
If he responds that he cannot control himself (which based on your narrative above will be the case), you should request a written doctors note that states that this can or will occur. Even then, simply because one has mental issues does not give them the right to break law or contract (in this case he is doing both). You could actually have him arrested for these activities.
In short, you can evict him, but you should give him a written warning that it is going to happen and will void his rental agreement. To that end, you may want to point out the section of the contract that he is voiding with specificity. One way to solidify the eviction would be to report the activity to the police when he is in the act. However, I can understand refraining from calling the cops as that is rather extreme.
Let me know if you have any additional questions or comments.
Best wishes going forward.