Nice simple game!
Being able to play as a super strong boss is a very fun feeling, and dishing out waves of bullets is very satisfying.
I really like that there are some upgrades to get after each fight, It really adds to the excitement to see what strong ability you will get next. The thing I would look forward to the most in future versions would be more abilities and upgrades to get, maybe even some paths to go down to get different builds.
I think that hitting the player feels a bit like luck most of the time, it could be nice to have some skills or abilities to force them to move in certain directions to catch them. Although, this may be an intention thing because you can dodge most boss attacks.
I also think that games like this should really lean into the generic tropes that are seen in boss fights. Having to worry about certain things within a boss fight that you don't usually do in normal games adds a nice gameplay dynamic. These things could be : The boss having a weak spot, Getting hit by things within the arena, player skills and abilities to get an advantage, etc.
The visuals are very nice, creating a cool underwater aesthetic.