Waterproof Locker (Subnautica)

The small storage cube is a 3x3 inventory storage box.

Crafting
Titanium x3

Although it is just 3x3 slots big in the inventory, the given space for storing items in it is 4x4 slots.

To store items into it, the storage cube must be placed somewhere under water. You can do this by dropping it. Then left-click on the top of the cube to open it and store your stuff. When you click on any other side of it you pick it up and the storage box moves back into your inventory.