I have a good number of turtles - all living outdoors (except the African Sidenecks, who have to come in during the winter - and HATE it.)
I have one very large box turtle, a Gulf Coast Box turtle, who, oddly enough, spends most of his time in the pond. He swims around, spends time totally submersed underwater, and even eats in the water. It used to freak me out, but he keeps on going in there, and is quite happy. He does spend time on land, too, but most of his time is spent in the water.