boat
艚 centers on a boat or vessel, specifically a type of boat. The meaning is straightforward and concrete.
艚 combines 舟 (boat) with 曹, which likely contributes the sound. The exact historical development is uncertain, but it follows a common phono-semantic pattern for boat-related characters.
The left side is a boat 舟, and the right side 曹 looks like a container or trough. Picture a boat that is like a floating trough: 艚 is a boat.
For ソウ, imagine a boat sailing on the sea: sea -> ソウ, and the boat 艚 glides across the water.