I'm really lucky because I live in a rural area, and I do find chances to swim/kayak, etc., in just trunks (swimming holes out in the woods, etc.). It's fun and all, but I know once the moobs are out there's no chance to pass if someone were to come upon me. Whereas, in a binder and tank top, I can pass.
I hate that word 'topless'--it just sounds so female-oriented to me. I just say, shirtless, took my shirt off, etc. Would a cis guy call it 'topless'? Just wonderin...