Do you have to wear a swim suit or is it cool to wear shorts and a t-shirt?

It is your choice.

You can wear whatever you're comfortable in. I wore running shorts and a sport bra. Most people have on some kind of bathing suits, but any clothing will do. A t-shirt is fine. Just remember that whatever you wear will smell like sulfur and it's hard to wash it out. Wash them separately and rinse your washer twice. Don't wash any other clothes with them because the smell will transfer. You may think you have them clean but you'll notice when you sweat, the smell will come back.