When a hero enters the Whirlpool, he loses 50% rounded down of the creatures in his army's weakest stack. The weakness is determined by total health of the stacks. With Sea Captain's Hat or Admiral's Hat (only in Shadow of Death), a hero can enter Whirlpools without losing creatures. To avoid losses, a player should divide one of the most disposable creatures to an individual slot, which naturally becomes the weakest stack and is therefore lost during the teleportation.
Whirlbools have six squares which can be entered or transfered to. This can easily cause a situation, where the boat cannot move outside the Whirlpool area after the teleportation. Note that a space key can used to re-enter the current square to avoid unnecessary movement.