Enter the following at the Start Game screen. After entry, press Start.

Unlock all Boats:
Press Up, Down, Left, Right, Left Trigger, B, X, X, Right Thumb Stick.

Unlock all Battle Modes:
Press Y, A, X, B, Left Thumb Stick, Right Thumb Stick, Black, White, Right Trigger, Right Trigger.

Unlock All Arenas:
Press X, Y, Up, Right, Left, Down, Up, Down, Left Trigger, Left Trigger.

Invincibility:
Press Left Thumb Stick, Right Thumb Stick, Down, Left, Down, Left, B, Y.

Unlimited Ammunition:
Press Black, White, L, R, Right Thumb Stick, Right Thumb Stick, Y, X.

Infinite Turbo:
Press Up, Up, Down, Down, Left, Right, Left, Right, B, A.

*The following cheats only work in multiplayer mode*

Blood Ball: Seaborn Soccer Mode:
Press X, Y, White, Black, B, A, Left, Up, Right, Down.

Drive Puffer Fish in Multiplayer Mode:
Press A, B, Black, White, Y, X, Right Thumb Stick, Right Thumb Stick, Left Thumb Stick, Left Thumb Stick

Hints

Fireshark Gunboat:
Beat the "Up The Nagau" level, under the ensign difficulty setting.

Guncat Catamaran:
Beat the "Protection Racket" level under the ensign difficulty setting.

Gunshark Gunboat:
Beat the "Ships In The Night" level under the ensign difficulty setting.

Jackal Devil Boat:
Beat the "Assault On Black Moon" level under the ensign difficulty setting.

Lightning Speedboat:
Beat the "A Friend In Need" level under the ensign difficulty setting.

Pike Speedboat:
Beat the "Payment Is Due" level under the ensign difficulty setting.

Salamander Speedboat:
Beat the "Fish In A Barrel" level under the ensign difficulty setting.

Switchblade Hydroplane:
Beat the "Gladiator" level under the ensign difficulty setting.

Tigershark Gunboat:
Beat the "Sampan Surprise" level under the ensign difficulty setting.

Clanbake Battle:
Beat the "A Poke In The Eye" level under the ensign difficulty setting.

Kingdom Come Battle:
Beat all levels in the "Baptism of Fire" act under the ensign difficulty setting.

Metal Massacre Battle:
Beat the "Hurricane Of Fire" level under the ensign difficulty setting.

Thanks to Revolution readers Aqualad, Andrew Barrett, RyanKingOfGames, and zugeta!