I gave up after being killed by Seth for about the 90th time with 581 fish
What I liked:
* Great style and pixel art
* Fitting music and sound effects
* Keyboard controls are good and can be adapted in the options screen
Suggestions for improvements (prioritized):
* On the continue screen, the "No" option should _not_ be the default option (make "Yes" the default or add a superposition state before the player has pressed any key), as novice players are likely to accidentally to choose this one
* Kill enemies that are depleted of energy immediately, instead of keeping them on-screen until you stopped shooting. Else players will get frustrated, as they're uncertain at what point an enemy is dead (especially if it's a new type).
* Provide a better possibility to anticipate, when Seth is going into a new rage mode (e.g. make him shortly stop in an invincible mode [e.g. panting] or provide graphical/numerical indicators within the energy bar, so that players can adapt their behaviour early enough without getting hit
* Do not reset killed enemies, if they're off-screen.
* Consider an option to leave a level, while keeping the fish in order to level up at the shop.