Just some advise from a genetic female. I obviously don't know anything first hand about knowing which bathroom to use, but I would say use the female bathroom/ changing area. Nobody should be looking at you while you're changing, and I'm guessing that there are private stalls for changing, right? As long as you are presenting as a female going in and coming out, there shouldn't be anything odd about that. But someone in a girl's swimsuit coming out of the men's bathroom, that might catch attention, where as someone in a girl's swimsuit coming out of the girl's bathroom is no different then anyone else!
I'm personally very modest, so there are genetic girls out there that wear swim suits that hide quite a bit! I wear a tankini (two piece where the top covers my stomach) or a one piece, and either of them with board shorts over, since I do not like my legs!
Any board shorts or swim shorts would probably work well for you, as they aren't too tight, and (at least this is the way mine are..) you wear your regular bottoms underneath the shorts, so that would probably help with keeping things in place.
But the most important thing would just be, stay confident and have fun with your friends!
Hope this helped at least a little, or even just gave you another perspective! Good luck!