Cannon Balls were added in the Naval Warfare Reborn expansion. They are used to load Cannons and can then be fired from Cannons. Cannon Balls can be crafted with Blacksmithing individually or by the sack. A minimum of 65.0 Blacksmithing is required to craft Cannon Balls and can be created in bulk (Sacks) at higher skills.
To load the Cannon Balls into Cannons. Double click the Cannon Balls in your inventory and target any single cannon on each side of the ship and wait. You can see how many Cannon Balls each cannon has by single clicking on a cannon.
After you've loaded your Cannons with Cannon Balls, you can then issue the "Fire Left" or "Fire Right" Ship Commands, this will give you a target reticle, allowing you to aim at an enemy ship, player or creature.
The Cannons will each fire one time, consuming a single Cannon Balls from each cannon. Note that Cannons are not entirely accurate and can sometimes miss.