flow
湝 centers on the idea of flowing, specifically of water moving in a steady, continuous stream.
湝 combines 氵 (water) with 皆, which likely contributes the sound and possibly the idea of 'all together' to suggest a broad, continuous flow.
Water 氵 flows together with 皆 (all), like a stream where all the water moves as one continuous flow.
For カイ, imagine a kayak gliding smoothly on the flowing water: kayak -> カイ, and the water flows steadily beneath it.