I found myself really engaged with this. Beat ten levels and was looking forward to deploying my newly bought barrel when the game ended :)
There was something very satisfying about blowing those ships out of the sky with a nice broadside. Some combination of the graphics and the sound really made it a nice experience.
Btw, game crashed for me when I was intentionally letting enemy ships by to see the game over screen. Super minor, but thought you should know.
Nice job.