Instant Win
Pause the game, hold A + B + Select on Controller 1, then press Select on Controller 2 to enable an instant win for every match.

Various Cheats
During gameplay, hold A, B, and Select on Controller 1, then press the named buttons on the named controller the named number of times (while still holding these three buttons) to enter some cheats.
Controller 2: Select x2      During 1-Player, when the ship sinks, the water will temporarily be green. In 2-Player, the water in the river will become light-blue and flow in the
Controller 2: Select x1      During 1-Player, when the ship sinks, the water will temporarily be red. In 2-Player, the water in the river will become purple and flow in a reverse
Controller 2: Select x20   In a 2-Player game, at the guillotine screen a different animation will occur for the execution.
Controller 2: Select x5      Press Start to enter the pause screen and then return to the game. This will complete the current map during the Selecting Castle phase.
Controller 2: Select x6      Press Start to go to the pause screen, press Select, and then return to the game to instantly finish the current phase. This doesn't force success.
Controller 2: Down x5      Press Start to hold game without hiding screen, and twice to view normal pause screen.
Controller 2: Select x13   The next game starts from the last battle.

【Not personally tested】