Hydro Thunder cheats and cheat codes for Dreamcast

Unlock Bonus track:
Highlight "Back" at the options menu and press 
Up, Up, Down, Up, Down, Down, Start. 

Full pause screen                                                 
Press X + Y, while paused the gameplay.

Unlock fishing boat:
Unlock all the tracks and highlight Thresher at the Boat Selection
screen. Hold L + R and press View twice. 

Faster start
Before the race begins, hold L during the countdown. 
when the countdown reaches 3: Release L and hold R 
when the countdown reaches 2: Release R and hold L 
when the countdown reaches 1: Release L and hold R 
Note: This trick works if you have a default controller setting.