back-water, eddy, whirlpool, flowing water
洄 centers on water moving in a circular, returning motion: a backwater, eddy, or whirlpool. The broader sense of flowing water is connected to this swirling, reversing movement.
洄 combines 氵 (water) with 回 (to turn around, return), suggesting water that turns back on itself. This is a standard phono-semantic compound where 回 likely also contributes the sound.
Water 氵 that turns around and returns 回 creates a swirling eddy or whirlpool. Picture a stream where the current doubles back on itself, forming a circular backwater.
For カイ, imagine a kayak caught in a whirlpool: kayak -> カイ, and the swirling water spins the kayak around.