pure water, clear water, pool in a river
瀞 centers on water that is still and transparent: pure, clear water, especially a calm pool in a river where sediment settles and the surface becomes mirror-like.
瀞 combines 氵 (water) with 静 (quiet, still), suggesting water that is calm and clear. The character is a phono-semantic compound where 静 likely contributes both sound and the idea of stillness.
Water 氵 plus stillness 静 gives a pool so calm and clear you can see right through it. Imagine a quiet forest pool with perfectly pure, transparent water.
For セイ, picture a serene, clear pool that is so still it looks like a sheet of glass: say -> セイ. The calm water says 'sei' as it reflects the sky.