Getting the last star for ships purchased requires repeatedly downgrading and upgrading a ship, and there's really no other reason to do that - it might save some time to make the number of ships required for the star the same as buying all the way up the upgrade ladder.
My only problem with this type of game is when the mouse controls make my ship fly wild when I'm low on the screen and inadvertently move the cursor out of the box.
Movement is a little too fast. Also did you know that the Mythbusters showed that bullets become ineffective after going as few as 6 feet underwater? So how do these hurled harpoons hurt me when I'm 40 meters under?
I wish I didn't have to tell my melee troops to move to attack someone who is range-attacking them from within their move radius. They just stand there getting zapped. Idiots.