I have very sensitive skin and everytime I go swimming I end up with a terrible rash on my bum…

I have very sensitive skin and everytime I go swimming I end up with a terrible rash on my bum cheeks where my bathing suit sits. It’s awful! I just get rid of it and it starts up again because I refuse to not miss out on summer activities because of it. The only thing that gets rid of it is hydrocortisone cream. I’ve tried different bathing suit materials and nothing works. Anyone have any suggestions as to how to prevent this?