HOW TO NOT GET A RASH?

every year i go camping at the beach for the weekend, im at the beach swimming for like 7 or 8 hours in all, everyone always gets rashes on there thighs near there genitals, and sometimes the rash is on the genital, wen i wear boxers i get a scrotum rash, wen i wear tidey-whiteys i get a thigh rash, how can i prevent it? or heal it? baby powder?