The Cannon is a one time per battle weapon that deals damaged based on the current HP of the enemy. It is dropped whole or, more commonly, in parts by the Evil Pirate.
Due to it scaling with the health of the enemy and the Evil Pirate being the only consistent source of Iron Cannonballs, it is not an ideal weapon early game.
After getting 50 Pirate's Gold, Gold Cannon Blueprint, A Cannon, and Level 100 Crafting, you can craft Gold Cannon.