Probably a one piece unless your going for sexy or very silly
Kneesuit or triathlon suit (with some leg coverage) so that 1) your package does not come out on the sides, and 2) you look like you are a sportsman wearing your unisex sports attire, but in the heart you know you are crossdressing. But if you want to be really bold and cause laughter, go for the swimdress!
I wear a womens one piece skirted swimsuit to the local pool
The birthday swim suit. That will let everyone know what kind of person you are. (Male person or female person is what I mean)
I have worn a one piece waterpolo suit to several water parks without any issues.