Note: This trick will not work on v1.1 of the game. Select stunt mode and ride in Dolphin Park. Go through all the rings and perform the following stunts:

Handstand (off throttle, [Down] and [Up])
Spin (off throttle, sweep Analog-stick clockwise)
Stand (off throttle, sweep Analog-stick counterclockwise) and Somersault (hold [Down])
Roll left (off ramp, press [Right] and hold [Left])
Roll right (off ramp, press [Left] and hold [Right])
Flip (off ramp, hold [Up] and press [Down])
Dive (off ramp, hold [Down] and press [Up])

A dolphin will squeak as you cross the finish line to confirm correct trick entry. Exit, select championship mode and the warm up option. Hold [Down] on the Analog-stick when selecting a character. When game play starts, your character will be riding on a dolphin. It can perform a Flip, Barrel Roll, and Submarine stunts. Note that the dolphin may only be used within this area. This secret is saved with your game, so that the stunts do not have to be repeated.