Intentionally run out of lives while playing the game. Then at the high score screen, press A, B, A, B, Select, Start. A cheat option will now appear on the main menu.
While playing the game, press A, B(2), Left, Right, Up, B, A to stop traffic and keep the turtles from sinking. If you entered the code correctly, a traffic light will appear.
Note: You must have a 2.1 or higher version of the Game Shark to use these codes in a Game Boy Color system.