Cool game and easy to learn (especially given the fact u had a tutorial in the game). The controls are taking a bit to get used to, especially when your ship is facing downwards and suddenly left is right when shooting :). The best tactic for me turned out to be "camping" (waiting for the pirates to approach and shoot them there), which actually takes away from the fast-paced gameplay you went for. The pirates (for me at least) were too fast and accurate. Tweaking these parameters (speed & accuracy) a bit might give you a way to better create a smooth difficulty curve.
Nice work!