slush, mud
淖 centers on wet, soft, messy ground: slush or mud. Both meanings describe a semi-liquid, dirty mixture of water and earth.
淖 combines 氵 (water) with 卓, which likely contributes the sound. The exact historical development is uncertain, but the water radical fits the muddy, slushy meaning.
Water 氵 beside 卓 (a shape suggesting something standing out or prominent) can be imagined as water mixing with earth to form a prominent, messy slush or mud.
For ニョウ, imagine a new (ニュー) layer of slush forming after a storm: new -> ニュー, which closely matches ニョウ, and the slush is fresh and wet.