Since sharks spawn at random in deep water, head out into the ocean with a boat or jet ski and watch the mini-map until it turns completely dark blue. Dive around until you see a red dot on the radar, indicating a shark, then jump into the water and wait until it kills you. It may take some time if the shark does not see you right away. After getting killed by the shark you will get the "Out Of Your Depth" hint. If the shark attacks and destrosy the boat or jet ski you will also unlock the trophy.