A Muslim is not allowed to go to a mixed swimming pool under any circumstance.
Question: Is it permissible for a Muslim man to go to a mixed swimming pool with the knowledge that the women there swimming suits form and would not listen to any admonishing?
Answer: Although looking without bad thoughts or lustful intentions at the women who are indecently dressed (and who would not listen to you if you wish to admonish them) is allowed, yet based on obligatory precaution, going to such places is absolutely forbidden.
1. Ayatollah Seyyed Ali Sistani, Question & Answer, Looking.