Land on water with Leviathan
Go to Easter Bay Airport or the abandoned airfield. Take a Levithan (the large gray helicopter) to any body of water in the area and press X. You will begin to descend. Continue until you are on the surface of the water and press Y. Your helicopter will have landed on the surface of the water and you can swim around. Do not let the helicopter go out of your vision range or it will disappear and spawn back to the airport.