I live in MD and used to keep box turtles as a kid. Just for a few months at a time and then released back to the wild. I have seen them swim, but not often and certainly not as well as a water turtle.
That's not quite true. They are considered semi-aquatic as hatchlings, they swim and can eat under water but as they get older and their shells dome, they float. They still love the water but without webbed feet, there's a whole lot of paddling and bobbing but it doesn't get them anywhere. Super cute to see though ;-)