Very cool concept and fun game. Best part of the game is the art.
I think it is very interesting that they added an extra dimension by going under water. Made me take a while to get used to dodging some attacks, but it was very fun. The only suggestion I'd make is that they could have timed the attacks for more intersting or challenging combinations, for example in the second boss, there are some times where the attacks are under water but the tentacle is above, so there is no reason to go underwater and risk getting hit making it trivial. It can happen with the last boss, but he has more mixed AOE/bullet attacks. Maybe even just alternating 1 above and one below in the same pattern could help.
But overal very good work, i'd like to play a full version of it! :)