This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
淛 combines 氵 (water) with 制, but the exact historical development and original meaning are uncertain.
The water radical 氵 beside 制 suggests something related to water, but the specific meaning is unclear.
For セイ, imagine a calm lake where you say 'say' to the water: say -> セイ, and the water 氵 listens.
—