if you go to the game cross town checkpoint and choose San Fransisco 1. Go to the first checkpoint and there is a hedge far back and sandpiles and go fullspeed right to one of them and you will use it as a jump right over the edge and in the water!!!
If you want to cheat, you are wasting your time because I have tried to put cheats in but they will not work. If you want to select your cars for take a ride mode, I suggest you have to complete the undercover mode.