Amazing work on the sprites, and I like the background music. I also like that you added in different types of ships with different shooting patterns. The dodging is fun and I like how the enemies move and shoot differently from each other.
The bullets blend in with the background too much sometimes. This is especially a problem in the first stage, where it becomes really hard to see them over the water when they turn blue. I would add a thick outline around them, or just make them bright red or something to contrast with the darker look. I also think the boss takes way too much time to come - I thought it was an endless shooter at first until I saw the screenshots of the bosses. Finally, there should be some audio cues for when you get hit.
Overall, great job on the art and the game! I managed to beat the whole thing.