Just finished playing. Overall a very good experience, although there are some rough parts.
First boss is near perfect, although can be a bit harder.
Second boss just broke for me and only attacked while I was under stone ark. For my friend the boss worked, but he ended up staying between rocks and shooting him, also he complained about lack of attack indicators, for example he suggested adding specific sound to tentacles.
Third boss is visually brilliant although attacking it is hard which makes fight way too long, you can shoot 20 harpoons and hit just one, also first phase can be a bit confusing.
Fourth boss was very disappointing for me. While this fight can be very epic, bad technical execution led to frustration, I've spent about 15 minutes trying to figure out how to start the fight, then got stuck in the boss, and later with his speed it was pure frustration trying to cut in front of him while getting cannons down. My suggestion - make him slower and somehow fix boat getting stuck in him and overall being uncontrollable near him due to waves and his size.
For future suggestions:
More mechanics like ship shutdown(e. g. permanent health damage that can be reverted to healable if you stunt yourself but you have to get it in the right moment otherwise you will be damaged even more),
More fight mechanics, probably something more than harpoon, because this game really holds potential for many mechanics that you should balance during battle.
Double bosses. Honestly I hoped fourth boss would be like two first but with more attacks for example. As proof of concept I suggest checking out Ultrakill level 0-5 playthrough to see how the concept can work well. (Also check out 7-4 level playthrough to see what I use as reference when thinking about improving fourth boss)
Hope you would think about implementing at least some of those, and thank you for making this game.