On Player 2's controller, hold Up, Left, and B, then reset the console. Continue along into the game. When you play, pause the game, and unpause while holding a button for a certain effect, as below.

Effect	Code
A (Diving Mode)	Exit diving.
A (Map Mode)	Instant "You've hit Jaws!".
A+B (Diving Mode)	Extra nine lives.
B (Diving Mode)	Instant killing of Jaws; proceed to boss fight.
B (Map Mode)	Instant "You've hit something!".