This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
洦 combines 氵 (water) with 百 (hundred), but the historical origin and intended meaning are unclear. It may be a phono-semantic compound, but the exact development is uncertain.
Water 氵 beside a hundred 百 could suggest a hundred streams or a vast body of water, but the true meaning is uncertain.
For ミャク, imagine a hundred 百 streams of water 氵 merging into one mighty river with a sound like 'myaku' — a made-up word for the rush of water: myaku -> ミャク.
—