KoKo, do you ever swim when grandma is around? If so, maybe you might keep the one-piece suit for those rare times, just to forestall more fights.
It's not as if she's asking you to wear a full diving outfit that covers you from wrists to ankles, after all.
What I'm hearing from you in these stories about your grandmother is that she thinks you can do nothing right, and YOU think SHE can do nothing right. You've got to back off.
Why do you have to back off? Because SHE isn't going to back down, and someone has to. Is that unfair? Yes, but you know what? It will be a lot easier in the long run if you let these stupid little control issues go, let her win a few fights, and allow her to relax a bit and feel she has some influence in your life. In order to change the way you two relate, you're first going to have to change the way you relate to her. Right now, your entire relationship is one circle of nonproductive anger patterns. Do you REALLY want that stress every time you interact?
Part of growing up is knowing when to walk away from a dogfight. Three years from now, what do you stand to lose if you give in now? Three years from now, what do you stand to have gained? It's negligible. Let this go. Write her a nice note. Wear the stupid swimsuit when she may see you. Quit taking every criticism of your life as a knife to the heart. Find some common ground with her. Once you stop being eternally defensive, you may find she stops being continuously offensive.