Pretty good.
Maybe you could give us a variety of enemies and have some of them shoot back. I couldn't help but think, "Where are all of these lifeboats coming from?" Also, Make the quicker moving ships blue or something, so it's obvious that they're too fast to kill starting from the middle.
My second time through, I clicked faster. I'm not sure why I was waiting so long before shooting again, the first time through, lol. If I had played it again, I'm sure that I could have improved my score.