Hi, all. I heard about this swim gear for FTMs/butches who have not had top surgery. It is an alternative to the Underwork's one.
http://www.danae.info/en/female-male/swimwear
I would suspect that the Trans-vormer 305 would give the best compression as the zipper in the back would ensure less stretching. The only thing is, it is not common for a swim suit to have a zipper (especially those worn in the U.S.)! I noticed it looks like it has shorts, but the description says it is without shorts, so I guess they just put them on the model.
What I personally wore before top surgery was a women's competition swim top under a
loose fitting sleeveless rash guard. The swim top didn't bind, of course, but the loose rash guard helped with disguising my chest. I will still wear the rash guards even though I have had surgery. On the bottom I wear board shorts.
http://www.rashguardshirtco.com/product/4106.html
Your mileage may vary on the items above, just thought it may be helpful.