hit a serpent with it and you can take command of your boat to drag the serpent to shore , jump off the boat and drag him up on shore for the kill ....a message will say you harpooned him and a rope will appear between you and the serpent , switch weapons and it will let go so you gotta be pretty quick , if the serpent is pulling you it will drain your stamina and the harpoon will let go , so you may need to slow the boat while dragging him , with a healthy long boat he wont destroy it quickly but likely need repairs after - ive got a whole stack of scales this way
people also use it to drag stone golems into holes but i prefer to just smash them with an iron mace