This is great! Ok so I just replayed to record what I thought was a bug but now I realize is just the fish requiring 3 hits to kill. I thought my harpoons were just getting destroyed. So I would highly recommend looking up what squash/stretch is, which is what I meant rather than a death animation. I think if you add a slight scale up + scale down, plus flash the fish red or white, or even show a little damage number popup it will be juicier with better feedback for the player to know they are dealing damage to fish. Squashing and stretching can just be done thru code. Im not sure if a death animation would be necessary as the game is very cute and it could seem a little harsh.
This is my favorite game feel guide as it really shows so many great ways to add juicy feedback: https://deepnight.net/games/game-feel/