Only a vet can diagnose the turtle we can only guess. Do you know any history of the turtle ie. How long has it had this condition? Here are a couple of guesses,it could be a swim bladder issue, also it may be a dietary issue, but it can be something that particular individual just does,or a past injury. Does it have sufaces to climb on instead of swimming muds/musk aren't strong swimmers & most swim akwardly anyway. The only sure way to know is get a vet to check.