sea mist, vapor
瀣 centers on atmospheric moisture over water: a fine sea mist or vapor that hangs in the air.
瀣 combines 氵 (water) with 韰, which likely contributes the sound. The exact historical development of the right component is uncertain, but the character is associated with mist or vapor.
Water 氵 beside the complex shape 韰, which resembles a tangled net or web. Picture a net of fine water droplets suspended over the sea: a sea mist.
For カイ, imagine a sea mist so thick you can fly a kite in it: kite -> カイ, and the kite disappears into the vapor.