Long story short, I'm both. But one at a time. For the sake of this conversation I've got my vagina to full blast.
Anyway, jeans and a t-shirt are exactly the kind of thing you're supposed to wear to a first date, especially a casual one. You might want to bring some spares, because spending the entire day in a wet swimsuit is like getting on the train to the sovereign nation of Chafencrotchen, and no one wants that.