still water, placid, tranquil
澶 centers on water that is calm and undisturbed: still, placid, tranquil. The meanings all describe a peaceful, motionless state of water.
澶 combines 氵 (water) with 亶, which likely contributes the sound and may carry a sense of spreading or extending, possibly suggesting water that spreads out calmly.
The water radical 氵 beside 亶 suggests a wide, open expanse of water. Picture a vast, still lake with no ripples, perfectly tranquil.
For セン, imagine a serene scene of still water: the word 'scene' sounds like セン, and the peaceful scene matches the kanji's meaning.