Save the game before attempting the following. Go into the ocean from anywhere and start swimming, preferably away from land. After a while a giant shark should will appear and eat Tony.

This usually happens along South Beach and North Beach. Swim far out in the water and stop. A cinematic will show Tony treading water until a shark jumps out and clamps on to him. Note: Make sure you do not have any dirty money or drugs in your possession because you will lose them.