swift current, rapids
瀨 centers on fast, turbulent water: a swift current or rapids in a river.
瀨 combines 氵 (water) with 賴, which likely contributes the sound ライ and may also suggest the idea of relying on or depending on something, perhaps the force of the water. The exact historical development is uncertain.
The left side 氵 is water, and the right side 賴 looks like a complex structure. Imagine water rushing over a tangled, rocky riverbed, creating swift rapids.
For ライ, picture a raft lying sideways in the rapids: lie -> ライ, and the swift current tosses the raft around.