Not bad.
I liked this game it shows good potential. Explosions would greatly improve this game. Don't let the life bar refresh when you face the boss. This will make the game way too easy. This game needs a lot more sound effects. Like I mentioned earlier you could add explosion sound effects and also add sound effects to the bosses cannon. Choose the sound effects wisely. The cheaper the sound effect, the cheaper your game. It takes talent to make a working game like the one you just did. With a little work you can have yourself a winner!