Now that is a very very cool take on the theme. I love the idea that after you beat the boss your old ship becomes the next boss. Very fun game, and the ship building is really cool.
I do think maybe a spawn timer before a boss spawning would be nice, right now if you build a super OP boss you are kinda doomed without so much as a head start.
Had a lot of fun with this one, it has some excellent mechanics.