Is he swimming or submerging and walking on the bottom?
I had a gulf coast box turtle years ago, and he would swim sometimes. I had a large kiddie pool as an outdoor enclosure for sunning when he was a baby. He'd swim from island to island. I don't have the pics anymore...
Swimming under the rock ledge...he hunts for hours. If he can't find a crevice to stick his head out of to breathe, he comes out then heads back under.
I put 2 pots together and spun them to misalign the holes in the bottom to stop the wind from blowing thru. He seems to favor this local in colder temps.
My box turtles swim...
they swim out to this floating fence board, when the pond is full
and my 50 pound Sulcata LOVED to swim...
just thought I'd share...and that box turtle is simply beautiful