Alternate character costumes:
Highlight a character and press the Z button for an alternate costume.
Source: planetgamecube

Change Replay Camera Angels:
During any replay (from time attack or championship)
Move the ''C'' stick (the right analog stick) in different directions to change the camera angels.

-C Up: ''spectator camera''.
Different random angels. (like opening demo)

-C Right: Rotates around the jet ski

-C Down: Water level on the left side of the jet ski

-C Left: First person view

Dolphin Park in Stunt Mode Normal:
Open the password screen(When on the Main Menu, choose Option and press X, Z, and Start)
Enter: ''463YWNX3''.
Dolphin Park should show up in Stunt mode Normal.

Load screen fun:
When a track is loading you can mess with the water by moving the analog stick.
Source: codes.ign.com

Password Screen:
Enter the options menu and press Start + Z + X to unlock a ''Password'' option. Use this option to enter passwords to compete in official time attack challenges.

Play with start screen:
When the start logo appears on the title screen, use the left thumbstick to move the start logo around the screen (which will cause the water to ripple where it moves).

Ride a Dolphin:
To ride a dolphin in Wave Race: Blue Storm, enter this code at the password screen:
DLPHNMOD
You can now ride a dolphin in Free Roam mode.
Source: Planet GameCube

Secret Island:
Enter Dolphin Park on free roam and go straight until you get to the hill sticking out of the water. Look slightly to the left and you'll see a circle of bouyies. Turbo toward it and half way there do a dive to gain another quick turbo and you should get to it before the out of bounds timer runs out. In the circle you'll find huge waves high enough to jump off of.

Time Attack Tournament Password:
When on the Main Menu, choose Option and press X, Z, and Start. Then, select the Password entry screen.On this screen, enter this password: J784WMHF

If all of the above steps were performed correctly, the Ethnic Lagoon track in Normal mode of Time Attack should appear. You can choose any character and customization for the 3 lap race.

After you finish the race, enter your initials on the next screen. Write down your initials and the 27-digit/character code. If you submit this code in the contest by October 25, you can win up to 100,000 yen!! This is around $835 for all the Americans out there... ^_^ You can also win a copy of Super Smash Bros. Melee!!

Unlock All Weather Conditions:
To unlock all the weather conditions in the Time Trial Mode (without having to race each track on each condition), including the rare Storm condition, you have to complete the Expert Championship.

Third Place: Unlock all Time Trial Weather Conditions for Normal
Second Place: Unlock all Time Trial Weather Conditions for Hard
First Place: Unlock all Time Trial Weather Conditions for Expert

Source: The Cube Club

Unlock Aqua Maze:
To unlock Aqua Maze, beat Championship mode on Hard difficulty.

Unlock Cool Ocean:
To unlock Cool Ocean, beat Championship mode on Normal difficulty.

Unlock Expert Championship Mode:
To unlock the Expert (7 Days) Championship Mode, you must first complete the Expert (6 Days) Championship Mode.

Unlock Hard Championship Mode:
To unlock the Hard (6 Days) Championship Mode, you must first complete the Normal (5 Days) Championship Mode.

Unlock La Razza Canal in Time Attack:
Open the password screen by pressing X, Z, and (Start) SIMULTANEOUSLY.
Select Password and type in the following:
MJV8LKL6
La Razza Canal should now be available in Time Attack mode.
Source: Gamepro

Unlock New Courses (US names):
To unlock the Arctic Bay course, you must play and complete it in the Normal Championship mode.

To unlock the La Razza Canal course, you must play and complete it in the Advanced Championship mode.

To unlock the Strongwater Keep course, you must play and complete it in the Expert Championship mode.

Unlock Strongwater Keep Course:
Complete it in Expert Championship mode to unlock Strongwater Keep Course

Unlock Trial Option in Tutorial Menu:
In order to unlock the Trial option, which is the ''???'' in the Tutorial menu, you must complete all stunts, jumps, and controls (Basic and Advanced).

Trial mode allows you to race for one lap alone in Dolphin Park, where you can perform as many tricks as you can before you reach the finish.

Unlock Victory Gate:
To unlock Victory Gate, beat Championship mode on Expert difficulty.