shore, shoal
沚 centers on a small landform at the water's edge: a shore or shoal, a place where land meets water and shallow areas emerge.
沚 combines the water radical 氵 with 止, which likely contributes both sound and the idea of stopping or standing still, suggesting water that has stopped or gathered at a shallow shore.
Water 氵 meets a stopping point 止: imagine waves gently lapping against a small sandy shore where the water becomes shallow.
For シ, picture the shore as a peaceful scene where you can sit and watch the water: scene -> シ, and the water stops at the shoal.