This guide will tell you how to find the shark and get the achievement by getting killed by the shark.
You will first need to get a jet and fly it to the end of the map. The jet will automatically break down once you fly it into the water, and there is a 99% chance of the shark spawning and killing you. You will earn achievement this way.
You can also go on a boat and reach the far end of the map. The shark will spawn in and get you killed.
There’s another way to do it: bring out your in-game phone and tap on Contacts. Click on the dial pad and dial the number 19997593255, which will make you Skyfall. This cheat will deactivate once you land in the water, and you can earn the Playstation trophy or Xbox achievement. After that, the shark will spawn, and you will get killed.