to boil, to wash, to cleanse, to soak
瀹 centers on using water to transform something: boiling, washing, cleansing, or soaking. The common thread is applying liquid to change or purify.
瀹 combines 氵 (water) with 龠, which likely contributes the sound and may relate to a vessel or pipe, suggesting water used for boiling or washing.
Water 氵 beside 龠, a shape associated with a flute or pipe. Imagine water flowing through a pipe to boil, wash, or soak something clean.
For セク, picture a section of pipe being used to boil water: section -> セク, and the water flows through to cleanse.