Yeah, I feel you. I was basically forced last year to wear female swimwear around my SO's family (not out to them, never coming out to them) and it was an okay time if I kept my shirt on, but then they got all uppity about why I had my shirt on at the beach, so yeah, I feel you. I ended up wearing a shirt half the time and the other half I just basically lied and said I was protecting my back tattoo (it takes up my whole back) from too much sun (even though I've had it for years and it's all black, sunscreen on it would have worked just fine) and they accepted the shirt after that for the most part.
The bad part is most waterparks and swimming pools I go to won't allow t-shirts, but rash guards you can wear, so I ordered the swimwear one from underworks. I haven't had any issues at the heated pool down the street wearing it, and I can wear it into the showers in the mens room, change in the shower stall to my velcro binder and no one's the wiser.
I enjoy swimming and the beach too much to not go, so I basically forced myself to deal with it, and my SO is supportive so when I needed to escape them last summer we would just go on a walk down the beach and ignore them. :p