Three-drops water radical on the left — the side-form of 水. It indexes 沿 in the water family with 河 river, 海 sea, 流 to flow. The original sense was tracking a watercourse downstream, which is why 沿 came to mean 'along' for any path that follows a line.
Upper-right top 几 acts as the phonetic top of the right half. Sitting over 口, it forms an old graph 㕣 (yǎn) — 'water flowing from a marsh' — which gave 沿 its yán reading. Treat it as a sound clue rather than 'small table' here.
The mouth-shape underneath completes the old 㕣 graph, originally picturing the mouth of a stream where water spills out. Stacked with 几 above, it reinforces the image of following a watercourse from its outlet — the core scene behind 'to go along'.
