water's edge, shore, bank
汀 centers on the boundary where water meets land: the water's edge, a shore, or a bank. It describes the narrow strip of ground right at the waterline.
汀 combines 氵 (water) with 丁, which likely contributes the sound. The character expresses the idea of a flat, even stretch of land by the water.
Water 氵 meets a flat, even stretch of land 丁: that meeting point is the water's edge, the shore.
For テイ, imagine a tea table set right at the water's edge: tea -> テイ, and you sip tea while sitting on the shore.