Before turning the game on, hold X and Y on Controller 1 and A and B on Controller 2, then press Power on the Control Deck to turn it on. You'll see "Debug Mode" on the screen. The following options will be available:
Adjust Fishing Conditions
Hold A on Controller 2 while on the Boating screen, then go to the Casting screen.
Before you get to cast, you will be able to adjust all of the fishing conditions, such as the Stage Number, Hour, Weather, etc.
Catch Random Bass
Go to the Casting screen and press B on Controller 2, and you'll catch random-sized bass, even some lunkers (a huge bass).
Catch Other Random Fish
Go to the Casting screen and press Y on Controller 2. You will automatically catch a random-sized fish other than a bass.
Catch a Huge Bass (Lunker)
Go to the Casting screen and press Up on Controller 2. You'll automatically catch a Lunker (a very large bass).
Speed Up Time
Go to the Boating screen and press X on Controller 2. You will see the time progress rapidly as long as you keep holding X.