The funny fishes can't even attack your back rank characters
So in order to get the treasure chest in the giant cave where you have to swim at least 8 squares I used a dirty trick to avoid using stamina potions and spells:
Put 3 characters who can hide to the front line (monk, ninja, bard for example), when stamina got low they kept turning around until they were attacked by fishes, the 3 front line characters hide and the fishes are helpless.
Now just keep on resting in battle and you can leave the battle with almost full stamina and swim more squares
3 times illogical but it works