raft, boat
艖 centers on a small watercraft: a raft or a boat. Both meanings point to a simple vessel used for crossing water.
艖 combines 舟 (boat) with 差, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a type of small boat.
The left side 舟 is a boat, and the right side 差 looks like a difference or gap. Imagine a raft that bridges the gap between two shores: a simple boat made to cross water.
For サ, picture a small raft sailing under the sun: sun -> サ, and the raft floats on the water.