What a wonderful game! I really enjoyed its concept, and it's incredibly creative right from the menu, haha. The idea of preventing the player from simply starting the game normally because they're the boss was brilliant.
The gameplay is simple and easy to understand, which makes the game very intuitive. Players can quickly grasp both the objective and how to achieve it. That's a great strength!
The ocean setting feels a bit dark and mysterious, which contrasts with the game's humor, at least from my perspective. Of course, this isn't a criticism at all—just an observation. The art style is simple and effective, and I'd say it gives your game a lot of personality. Because of that, it could be interesting to have some scenery above the sea as part of the background. Things like tropical islands, a beach, or even just the sky with a parallax effect could work really well.
Another thing I'd like to mention is the controls. I personally didn't have any issues with them, but it might be a good idea to offer a secondary option for selecting the fish's attacks, such as using number keys (0, 1, 2, etc.). The reason is that some players may have a broken mouse wheel, which could make the game more difficult to play.
The effects were subtle, but they fit the game very well and made the actions feel satisfying—especially the red trail the boss leaves behind. It evokes a sense of danger while also resembling blood in the water, a clear sign that this is a dangerous enemy (aka the boss, haha).
Congratulations on the game—it was a lot of fun to play. I wish you the best of luck in the jam!