hold (ship)
艙 centers on the storage area inside a ship: the hold where cargo is kept.
艙 combines 舟 (boat) with 倉 (storehouse), suggesting a storage compartment on a ship. The exact historical development is uncertain, but the component relationship is clear.
A boat 舟 with a storehouse 倉 on board: that storehouse is the ship's hold where cargo is kept.
For ソウ, imagine a ship's hold packed with cargo, and the crew shouts 'So!' as they finish loading: So -> ソウ.
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hatch; hatchway (ship)