mouse wheel: change melee/ranged weapon.

thank you! but also how do you fire the weapons?

Boats have 2 attack types. Melee and Range. You can switch between these two as balrasar said with your mouse wheel. If you see a number in the bottom right corner you have chosen range attack. To fire hold your left mouse key and aim. When you release the key you will fire and you you will see cannon balls flying.
Melee attacks you do the same way as range attacks with the difference you don't have to aim. But you have to be close to your enemy.
After you fired all your ammunition of range attacks you don't see the number but you still have to switch back to melee.

thank you so much!