Its good, but it needs work
I like the game system. Its easy to use, well designed, and nice looking. The music is very good as well.
Unfortunately, it has a really annoying flaw, and a couple of minor ones.
First, and most important, is camera drift. If you and another boat are trading shots right against the edge of the map, the camera has a tendancy to drift away from view until you don't even know where you are anymore. You should upgrade it so that the camera stays centered on your craft at all times. This is incredibly irritating to deal with as you must physically manuver your ship to get the camera to reset to where you can see again.
For some small improvements, I suggest adding a trading system or missions. The game is a fun combat simulator, but that's all it is. It has the potential to be a lot more.
You did do a really good job on the game engine though. Make a few minor improvements and fix the camera problem and it could be top 50 material.
Oh, and for people complaining about difficulty?
Upgrade your guns. Seriously. The first thing you want to concentrate on is longer ranged cannon. If you can sit back and shell someone without getting hit yourself, you can pretty much win any battle if your ammunition holds out. Hell, Nelson did that at Trafalger.