river bank, water's edge
涘 centers on the edge where land meets water: a river bank or the water's edge.
涘 combines 氵 (water) with 矣, which likely contributes the sound and helps express the idea of a water's edge or bank.
Water 氵 meets the shape 矣, like a river lapping against its bank. Picture the water stopping at the edge.
For シ, imagine a river bank where you see a single sheep drinking: sheep -> シ, and the water's edge is where it stands.