confluence of two streams, bend
汭 centers on the meeting or bending of water: a confluence where two streams join, or a bend in a river.
汭 combines 氵 (water) with 内 (inside), suggesting water entering or bending inward, which fits the meanings of confluence and river bend. The exact historical development is uncertain.
Water 氵 flows inside 内, creating a bend or meeting point where streams converge.
For ゼイ, imagine a lazy river bend where you say 'zay' (like a relaxed 'say') as you drift along: zay -> ゼイ.