The Golden Turtle is the thirty-fifth treasure that can be found in Uncharted: Drake's Fortune.
The Golden Turtle is found in Chapter 13 - Sanctuary?. This one is quite easy to miss. You can get it in two ways: 1) After getting off the jetski, jump into the water and swim to the ledge behind the waterwheel and go behind the waterwheel. The treasure is on the small ledge against the broken bridge. 2) After defeating the pirates, jump from the bridge near where it's broken, to the ledge below.