GTA 5 is a vast sprawling world full of missions, games within the game and its famous random character interactions. One of the coolest things within the game is to borrow one of the jet skis along the coast and head out into the waves.
You jump on a jet ski in the same way you enter any vehicle and then you are away. With the incredible attention to detail in the game, there are breakers to play with as you can jump off them and surf down them. If you time it right you can also do jumps off the rocks, although the jet ski will eventually get damaged if you hit the rocks too hard.
Watch the video to see the jet ski in action.