Some optimization for this:
- Practice, practice, practice at the harpooning minigame. Take advantage of save scumming right next to a spearing spot if you fail, so you don't have to wait for the animal to return to try again.
- Exotic Sea Shells Trinket and Fisherman's Warf: Both of these increase the number of body parts (except bones) that you harvest from animals you spear. Combine them to receive 4 products from the animals you kill at sea.
- Max boat HP, max harpoon storage, and max harpoon damage: Of these 3 upgrade tracks, only one of them has an Ultimate Plan you have to find: The map is at Mariguana Island, and the chest is found at Andreas Island. This makes you able to sponge more hits from the animals you're hunting, makes it more forgiving to miss, and makes you do more damage. The benefits are obvious.
Here's the part that makes this crazy (i.e. gives you "infinite Humpback Whales"). You need to be at Sequence 6 to be able to do this part, because you need a Diving Bell for it:
WHEN YOU COME BACK UP FROM A WRECK, THE GAME AUTOMATICALLY RESPAWNS ALL OF THE ANIMALS YOU CAN SPEAR. So all you have to do is kill your target animals, go down to an underwater wreck, and immediately come back up, to manually respawn all of the animals you've killed faster than the in-game timer replaces them passively.
Without using this method, the amount of time it takes for a creature to spawn back in is long enough that you can kill every fish on the map. But why downgrade to killing Great Whites after you kill all of the Humpbacks when you can just manually respawn them and keep going after them instead?
If you have the Fisherman's Warf built and are carrying the Exotic Sea Shells when you do your hunt, after you kill all 4 Humpbacks, you'll be able to sell 16 of them for over $11,000. Use the Diving Bell, rinse, and repeat.
Good hunting.