There is a bog difference between sex in a public pool and sex in a bathtub when you are already clean.
Public pools are very heavily chlorinated for a reason. The chemicals can be harmfull to your insides when pushed up rather than by just imersion. The dirt from other people that have been in the pool is not good either.
The best way to have water sex is for both of you to take a shower, then fill the bathtub with clean water. Water sex (and oral sex) is a great experience, but only in clean water with clean bodies.