The Cannon is a game feature used on ships, especially by NPC ships, to attack the player.
When the player steps next to one, they instantly sit on it and enter the shooting idle animation. On the cannon, a red "sight" will appear and the player will be able to move it using WASD. A small menu also appears, allowing the player to choose the desired cannonball to use. After that, they can click to shoot.
Below is a list of implemented and unreleased cannonballs.
Cannonballs[]
Currently, there are only 2 types of cannonballs that can be used by the player. Those can be bought from Marguan at Town of Beginnings.
Other types of cannonballs are currently unused or unreleased. Their price and location are unknown.
Normal[]
A normal cannonball that deals 15 damage to the enemy ship.
Can be bought for 
175 per 10 units.
Shotgun[]
A red-ish cannonball that splits into 10 normal cannonballs, similar to a shotgun shell.
On older versions of Grand Piece Online, it was called Scatter Cannonball.
Can be bought for 500 per 10 units.
Nuke[]
An unused and probably scrapped cannonball most commonly used by Phoeyu as an admin spec.
It works just like a Buster Call, being able to destroy an entire island.
On older versions of the game, it would destroy every single building and tree, but as that feature was scrapped, it currently only destroys all trees and likely kills all NPCs and players.
Firework[]
An unused cannonball that doesn't appear to have ever been showcased.
It may work similar to a mortar, being aimed upwards and then spitting into various normal cannonballs to look a firework, pretty similar to the in-game Shotgun Cannonball.