I'm glad the episode brought this up, that people can't tell the difference is... Well, I guess I should see it coming but I rarely do.
And then someone on a MLP site's comments said that terrapins are ALL freshwater turtles, including (their own example) SNAPPING TURTLES.
Trust me, those crocodile wannabes are NOT included in the term!
EDIT: Did some more research and apparently some tortoises HAVE been observed floating so that they can doggy paddle a little. The general agreement, however, is that most tortoises will sink like a rock. Those crazy testudines!