Your Inventory can be opened anytime your are on land by pressing the "i" key on your keyboard. There are three separate inventories in the game.
Scout's Inventory Edit
Scout starts off with a max inventory space of 12. You always have access to this inventory.
Aesop's Inventory Edit
Aesop has an inventory as well. Like Scout's inventory, it can be accessed by the player at any time, however the items Aesop carries do not show up under quick select. Has a max inventory space of 6.
Raft's Inventory Edit
This inventory can only be accessed when standing near The Raft. Has a max inventory space of 8.
Increasing Inventory Space Edit
Max inventory space can be increased up to 28 using a Pouch.