When I lived as a boy, I had boobs even before the HRT. If you look around you will see that lots of guys do. I used to wear a rash guard and then like a mesh nylon sport top thing on top of that if I was feeling self-concious. I NEVER went topless. That and baggy swim shorts should be all you need.
Andrew, I still have my boy swim stuff and some tropical button down shirts I used for my cruise last year. They should be pretty baggy on you I think. I was going to donate them to goodwill, but I would be happy to give them to you instead. Just let me know how to get them to you.