Here are some possibilities depending on the context:
If you're talking about the weapon itself:
* Net: A net would be used to capture something instead of piercing it.
* Spear: A spear is a general term for a pointed weapon, but it's less specifically designed for large marine animals.
* Hook: A hook might be used to pull something in, but not necessarily to kill it.
If you're talking about the act of harpooning:
* Release: This is the opposite of capturing or killing something.
* Spare: This implies choosing not to harm something.
* Protect: This is the opposite of hunting or attacking.
Ultimately, the best antonym will depend on the specific context in which you are using the word "harpoon."