Looks fun, couple of things I would love to see you revisit, if possible.
The sky color combined with flying brown stuff is too hard to see due to the color combination, perhaps a darker more contrasting look will improve this? The ship can also be a little darker in their shadow area.
When firing, it would be tremendous help if cannon fires nearly instantly after pressing the key, and the bullets are more visible. The visibility issue can be improved if the sky color is altered.
Again, this is just my opinion, all of this could actually be intended features. In that case, I apologize.