I have a different idea, as I've had several perfectly healthy turtles display this exact behaviour. In each case it was due to aggression from another turtle, especially by males looking to breed. My big female Scorpion musk started doing that, never venturing into the water, it was only after I removed the smaller male Loggerhead musk that she went back to normal. Same with a large female Caspian pond and male Chinese Thread who were getting harassed by a male Reeves.