This reminds me of the time that I went to a public pool years ago. My breasts were very small but noticable, especially with a wet t-shirt on. And that's what I wore, a t-shift and some shorts. I was still presenting as male.
I think if there's any definition, even if they look like man breasts that some men have when they're overweight, etc, you shouldn't be exposing them. I wore a t-shirt, had no bra on, and felt uncomfortable because I kept getting these looks from people like, "what is going on here?", lol. After I got in the pool and came out, I had to insert my hands through the sleeve to cover my breasts because it was obvious I had some growth. Awkward experience let me tell you.