To get this trophy you have to swim into the ocean and get eaten by a shark.
The easiest way to do this is to go to the Fairground Ride (by taxi or by your own means) When there go down to the beach and there will be a bunch of jet skis, drive one into the ocean and when far enough hit r2 to dive. You should see a shark if you're deep enough, and keep swimming until he eats you.
Note* Make take multiple attempts as the shark may not get you that fast so you will have to go up for air. You can do this after the first mission.