J.P.
Is he teething? Whenever my son teeths, he gets diaper rash. And if the diaper isn't changed RIGHT after a BM, he can even get the sores. After trying various creams, a friend said that her doctor told her to just use neosporin. So, we use the butt paste on most of it, and the neosporin on the sores.
As far as swim diapers, most diapers have chemicals in them, and you then have the chlorine in addition to that. Have you tried washable swim diapers?