How do I get harpoons in Assassin's Creed 4: Black Flag?
Once you begin Sequence 03 in the story mode of Assassin's Creed 4: Black Flag you gain access to harpooning. This allows you to purchase harpoons and upgrades via the Jackdaw upgrade menu, as well as take a rowboat to designated hunting areas (marked on your map) and take down sea creatures such as sharks or whales. As a side activity, harpooning lacks the depth found in naval or land-based combat, but it's still good to know what you're getting into.
Do I have unlimited harpoons?
First and foremost, you cannot reload your harpoons mid-hunt. The number of harpoons you start with is all you're going to get. You'll be replenished after every hunting session, but the only way to increase the number of harpoons you can carry is via the Jackdaw upgrade menu. Compared to some of the other available upgrades to the ship and Edward's weaponry, it's fairly cheap to maximize your harpooning ability.
How do I hunt whales and sharks in AC4: Black Flag?
In many cases, harpooning is very similar to attacking opposing ships with mortar shots. You need to lead the target and anticipate where it will be by the time the harpoon reaches it. Once you land the initial harpoon shot, you can focus on targeting the base of the rope to make things easier.
There are two health bars present while you're harpooning. One is the health of the target, the other is the durability of your rowboat. More aggressive enemies, such as sharks, will attack the rowboat. A well-timed harpoon shot will lessen the damage inflicted, and potentially turn the beast away completely, but you're basically playing a game of chicken. You must kill the target before your rowboat is destroyed.
With the target down, you can use the materials earned from the kill to sell at the harbor master shops, or use for crafting new and unique items.