The Cannonball Crate is one of the three Supply Crates in Sea of Thieves. The Crate is portable, and can be filled with Cannonballs. It can be found on Islands at random, or acquired from a merchant, provided a pirate has a quest that requires one. The Crate is always found empty, and can be filled with up to 50 Cannonballs. Pirates can also withdraw Cannonballs from the Crate. The Crate can be sold or delivered to the Merchant Alliance in exchange for meager rewards. Supply Crates are only requested in Quests for Trade Goods from Messages in Bottles, and only one Crate is ever requested per quest.
The resource crates require large amounts of supplies, and the rewards are lacking, so giving them to the Merchant Alliance is not recommended. Pirates are generally better off using them as portable storage.
The Cannonball Crate can be placed next to the Cannons on a Galleon to drastically shorten the time taken to refill on Cannonballs. On a Sloop, the close proximity of the Cannonball Barrels makes this redundant.