Translation:Turtles live up on the ground and down in the water.
Yes, but we often use just vann when talking about something living in/under/over the water: 'i vann/under vann/over vann'. I assume this is because we're actually talking about water in a general sense, and so there is no need for the definite. You can think of it as saying 'underwater' instead of 'under the water'.
You can still use the definite forms as well; both in general, and in this sentence. :)