Like Letum mentioned, you can use your mouse wheel to zoom in, but if you roll the wheel slow enough, you'll notice that there are 3 different views you can choose from. I prefer using the middle position where you can see the whole gun. As your boat rocks up and down, you can watch and shoot when your gun is pointing close to the right position towards the ship.
You'll still miss some, but with a little practice, you'll get the hang of it. It helps me quite a bit.
