Hey this came out really well. The sprite work and most of the mechanical feel of the player are the best parts.
I only have a few pointers and one gripe.
First, I think the shooting enemies shoot too fast. Second, the sound of the bosses dying is way too loud. Third, the game slowed down massively in full screen for some unknown reason, so i just zoomed in the browser tab and it was fine. Lastly, the machine gun boss has way too short and indistinct telegraphs for its attacks.
My only gripe is that I don't think this fits the jams theme very well. It may be tangentially related by the players water gun and the chinchilla's hate water bit, but its toeing the line. I wish you luck with the judges on that front.
Hopefully the extension of your project is awesome, great job and hope I see you around!