I was just reading about them on wiki and it's very interesting. You would assume since they spend so much time in the water that they would lay their eggs near it. But as Porkpal said they probably want to protect the nest from flooding. Also it say that they don't enter the water until 21 days after hatching. If they enter to soon air can enter the turtle through the split left behind from the absorbed yolk sac, and kill it. Laying far from the water might help insure the young turtles don't inadvertently enter the water.
