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 by the beach. I used a boat from the marina and then I power boated over to the fairground area then went out further into the deep stuff. When far enough hit
to dive. You should see a shark if you're deep enough (little red dot in the ocean), keep swimming until he eats you. It may take a few attempts for him to notice you.
EDIT: If you are having problems getting the shark to come near you while you are in a boat try this. I was having a very hard time finding one while I was in a boat so, I just jumped out of my boat and floated for about 3 minutes and one killed me. Credit to Tweakleton for this edit.