Speedo Easy Short Sleeve Swim Shirt

NTA. It's common to wear a t-shirt swimming. Very common. Also, even if it wasn't you decided that you wanted to wear one. What if you'd decided that they had to swim topless because "everyone would be looking if you had no top and they do" and pressured them when they didn't consent to it? I'm willing to be they wouldn't accept "everyone will be looking" as a piece of logic. Also, you can buy swimshirts that literally look like t-shirts - like this one on Amazon Speedo Men's Uv Swim Shirt Short Sleeve Loose Fit Easy Tee Rash Guard Shirt. It's made by speedo, the company so ubiquitous with swimwear that people call swim trunks "Speedoes".