I've been swimming once intentionally and twice unintentionally.
The first time was intentional. I dumped myself out of my tube a couple of times a few yards from shore to see where the tipping point was and to see if I could get back on the tube.
At first I tried sort of lifting myself into the seat with the tube behind me, but I wasn't strong enough to lift myself out of the water, even though the tube was supporting me fine, so I switched to a face down orientation and pulled myself onto the tube seat as far as I could and then did a quick flip/roll to the seated position. It's not that hard to do.
The unintentional times were once in the surf and once in the mud. In the surf, I didn't get dumped out of the tube, I was backing into the surf and got knocked down. I should have been launching face first to begin.
The other unintentional time was at HH and I was out on the flats and I decided to go into shallow water and stand up for some reason and I was in about 2 ft of water but as soon as I tried to stand up, the mud squished out from under me but the fins caused me to topple backwards as my heels sank faster than my toes supported by the fin blades and just that fast, I was swimming. Oh well... no harm, no foul.
I've always felt that float tubes were sort of self-selecting and people not comfortable in the water wouldn't want to have one. But I've recently learned that there are tubers who don't swim and are afraid to be in the water and that seems crazy to me.
As long as you're not losing your gear, getting dumped out of your tube is nothing more than going for a swim and personally, I find swimming with fins really easy. Even at 66 years old and overweight. But that's just one crazy old man.
How about you? Ever gone for a swim?