The Ship's biscuit is an item that is obtained from redeeming 100 doubloons. It heals 250 life points and boosts a random stat by 2. This item is now a tradeable rare, doubloons are no longer available via Treasure Hunter.
The biscuit cannot be traded via the Grand Exchange however may be traded in-person, being the second ever tradeable rare not available via the Grand Exchange after the Magical eggs from another Treasure Hunter promotion.
After the promotion, ship's biscuits can be converted to coins, at 1000 coins per biscuit.