An unpredictable joker, taking his boat can lead to a number of different events.

You might:

• Get Taken to the Failboat
• Get Attacked by the ferryman, a lv16 monster: Defeat him, and you'll be rewarded with a loss of five turns as you drift aimlessly without a ferryman.
• Get Taken to the lair of the Psychadelic Mushroom of Eternal Chaos, which you then have to fight.
• Some good stuff, too, I guess.

This is all assuming you pay him. Its unknown what happens when you don't, as few are willing to upset a Joker.