vast surging waters
沺 centers on vast, surging waters—an expansive, powerful flow. The single given meaning is directly about water in a grand, overwhelming state.
沺 combines 氵 (water) with 田 (field), likely as a phono-semantic compound where 田 contributes the sound and water provides the meaning of vast surging waters. The exact historical development is uncertain.
Water 氵 flooding a field 田 creates a vast, surging expanse—沺 is that overwhelming flood.
For デン, picture a den of rushing water: a hidden den where vast surging waters roar—den -> デン.
—