This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
湒 combines the water radical 氵 with 咠, but the exact historical development and original meaning are uncertain.
With water 氵 beside 咠, imagine a rare, mysterious liquid whose purpose is unknown—its meaning is unclear.
For シュ, picture a shoe floating on the water: shoe -> シュ, but the kanji's meaning remains a mystery.