Great sighting Winston! I agree that it looks most like and is most likely to be a Green Turtle (Chelonia mydas), however the subject is too far away in the image provided to be positively identified beyond doubt. Even though it's probably Green Turtle, it could be another species of sea turtle, so I've entered it under the superfamily Chelonioidea of which it certainly is. It's definitely worthwhile keeping this sighting as a record.