I agree with Trippin and Bella. If he tries to flirt with you, don't be afraid to remind him he's a friend, nothing more. I don't know much about women's swim wear (we get those God awfully uncomfortable mesh crap pants and call it a day ), but the shorts Bella suggested might help ease your mind a little, and might suggest to him that you don't want to reveal too much (to him, at any rate).

Enjoy the beach!