This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
涗 combines 氵 (water) with 兌, but the exact historical development and original meaning are uncertain.
With water 氵 on the left and 兌 on the right, the shape suggests something related to liquid, but its meaning is unclear.
For セイ, imagine saying 'say' to a mysterious water character: say -> セイ, as you try to guess its meaning.