A few things to think about when selecting a swimsuit:
For the top, if you have breast forms, you need to select a top made for breast cancer patients with mastectomies. Normal tops either won't hold your forms in place resulting in an embarrassing floating form a few minutes into your swim. Also, non mastectomy tops will either be cut too low on the chest or too high at the armpit which can cause problems. (no cleavage, anyone?)
For the bottom, you need something that will hide your penis if you are pre- or non-operative. Do not use a bikini bottom or similar! As much as tucking might work on land the combination of heavy activity, the water, and the swimsuit's construction will make it almost impossible to maintain a proper tuck.
Skirt suits seem to be pretty popular for transwomen. A couple things to remember about them, however is that they, like bikinis and one-piece suits, highlight your lack of curves and that they aren't particularly popular with anyone but my grandmother and transsexuals. Especially if you are stealth a skirt suit on a young person can paint a bright red "I'm a transsexual" on your back to anyone who knows anything about transsexuals.
I'm not saying that skirt-suits are wrong for everyone, I'm just saying that they have their drawbacks.
Probably the best bottom wear are women's swim trunks or "surfer shorts" over a bikini. The bikini holds your junk in check and prevents exposure when the trunks fall off while swimming while the trunks hide your stuff while on land and provide subtle cues that make your hips seem more womanly.
Don't worry about your trunks slipping off when you are in the water. Even a half ass tuck, once underwater, would be hard to spot and nobody is going to berate you for wearing something underneath a loose garment.
Of course, if you're post operative or have naturally full-sized breasts then the best way to go would be with the standard low cut top and skin tight crotch. As they say "the proof is in the pudding" so if anyone thinks you're "really a man" all they have to do is look at your not-man anatomy.