This game reminded me a lot of River Raid for the Atari 2600, with some upgraded graphics and a pirate theme. There were a few problems that kept me from playing beyond the end of the first battle.
-Collision detection: After playing a few times, I noticed that something is a bit off with collision with objects, as in my ship would explode before touching another ship or obstacle. This is particularly problematic with the edges of the screen, where you ship will instantly explode even if there are no rocks present.
-Sound: The music and effects are okay, but it would be nice if the effects were consistent throughout. Cannonballs explode when they hit other ships or rocks, but not when they hit islands. This mismatch seems a little odd. It might also be nice to have a sound effect feedback for the cannon firing.
-Controls: I’m not a fan of the M key being the fire button. (I tried pressing space the first time I played.) While the key mapping is on-screen, it took me a bit to notice on first go. Same with the A and D keys to move, but they are a standard mapping. You might include in your author’s comments the key mappings as some players will look there while the game is loading. An instruction screen before the first play might also help.
-Clicking to start a battle and upon death: It was unclear at first that I had to hover over the letters until they enlarge before I could click to advance. Since you never use the mouse in the game, it is a bit awkward to have to click some text to restart the level.
-Ship speed: While you can control horizontal movement, there is no way to adjust vertical movement. This is sometimes problematic in narrow passages when being fired on by turret enemies, particularly when they are firing at you from the side, as you can’t avoid fire. Having W and S adjust the ships speed (i.e., the upward scroll speed), could remedy this. Giving the player the opportunity to adjust between three speeds (quarter, half, and full) would allow them more opportunities to evade enemies that simple horizontal movement cannot.
-Radar: I’m really not sure what this is supposed to show the player. At one point all the dots were below the compass and later they were above.
I’m not 100% sure why, but I felt like the play was more tedious than challenging. This made it hard for the game to hold my attention for long. Keep putting your work out there and incorporate the feedback that you get. I look forward to seeing one of your submissions on the front page.