I propose a slight change in how ammo is changed, simply that when another ammo type is selected It wont immediately reload but wait until the cannons have been fired and then reload with the selected ammo except when the button to select said ammo is held for about a second, changing ammo in the current way.
The reason I suggest this is simple, after firing a broadside or such as and then wanting to change ammo type you must start reloading again after already starting to reloading after firing. This will possibly add a few seconds extra until you have you cannons back and ready to fire, which could put you and your ship at an unfortunate tactical disadvantage.