Moonglow Bay

This is a story related boss fight. You get a quest to go to a limestone cave, and after a quick cutscene you gain access to it.

Inside of the Hidden Lake there is a whale with three harpoons in its back. The "fight" is simple enough; just follow around the whale until it stops, then cast your line at the harpoon. There should be a white outline when you should cast. Then fish like normally, pulling in the opposite direction, then there is a quick QTE.

Also make sure you aren't too close to the whale or your boat will take damage. But I feel the hitbox of the whale is large since I still got hit twice, but you can repair whenever you go back to town.

For the third harpoon I think my game softlocked, since the QTE would appear and disappear so fast there was no possible way for me to react. If this happens to you, I found that making a save and quitting back to the menu to reload restarts the boss fight. That attempt I had no problems.

After the third harpoon is out you get the achievement :)