A Water object, that instantly moves the hero to another whirlpool. 50% of the units(rounded down) in the weakest stack in the army are lost, unless the hero wears the Sea Captain's Hat or Admiral's Hat.
If you have a free slot, split of three of your weakest units to it. This will give you three 'rides' as cheap as possible. Since there are six squares on each Whirlpool, it's not unusual that they are placed in such a way that you can't get off of it, and have to take another ride.