The Sea Sack is a backpack introduced in the Shipwrecked DLC, used to expand the character's inventory. While it has only 6 inventory slots, it slows the spoilage of perishable items contained within. It requires 5 Seaweeds, 2 Vines, and 1 Shark Gills to craft and an Alchemy Engine to prototype. Sea Sacks function identically to Insulated Packs from the Reign of Giants DLC.
The Sack itself cannot be placed in one's inventory or a Chest, and only one may be equipped at a time. When a Sea Sack is created, it automatically equips itself in the Body slot, if available. This item uses the same equipment slot as Armor and Vests, which means wearing one requires sacrificing the benefits of the other.
Although Sea Sacks cannot be placed in inventory or Chests, they can be placed on the ground and used for storage. Holding an item over a Sea Sack that is on the ground will give the 'Store' option, allowing the Sack to be filled without equipping it.
Despite slowing the spoilage of Food items, the Sea Sack does not work exactly like an Ice Box. Unlike the Ice Box, the Sea Sack will not stop the spoilage of Ice and Ice Cubes and will not cool a Thermal Stone.