shore, waterside
漈 centers on the edge where water meets land: a shore or waterside. Both given meanings point to this same boundary idea.
漈 combines 氵 (water) with 祭, which likely contributes the sound. The exact historical development is uncertain, but the water component fits the shore meaning.
Water 氵 meets a ritual altar 祭 at the edge of the land: picture a shore where ceremonies are held right at the water's edge.
For セイ, imagine saying 'say' to the shore as the waves wash in: 'say' -> セイ, and the water answers at the waterside.