I love the humor of this game, I'm totally down with a fish accepting a deal from aliens. The visuals and music are nice, and the controls feel responsive!
Two pieces of feedback from me are this game definitely needs a health bar for the boss and player so you can measure your progress. Other than that I would look into either getting another hit sound effect or two, or modulating the pitch of the hit effect a little bit each time it plays, ideally both. When you have a sound effect that plays often, if you change it's pitch just a little bit each time the players ear doesn't get so used to it and it doesn't seem repetitive. I like to have my sounds pick a random pitch from 0.95 to 1.05 to start and adjust from there until it feels good for the sound.
Awesome job!