I let one of my tortoise swim once for about 6 seconds as an experiment to see if they could, after a TFO discussion here. Is there a reason you are swimming yours?
Just to see if she could and to soak the new growth on her shell. I don't usually swim her but like you tried as well Tom, I was curious. She actually just bobbed about and didn't kick much except to turn around when her face got close to the wall. Then I let it drain out so she was only really floating for a minute or two as I took the pictures.