CUHK doesn't appear to expound the semantic shift from wading in water to under water.
Axel Schuessler, ABC Etymological Dictionary of Old Chinese (2007), p 425. But this redirects you to p 303.
I don[t understand the red underline, which can't refer to dead humans, because dead humans who "lie at bottom of water", aren't normally construed to "be soaked (in pleasure)"...unless you're a sadist! Living humans can't lie at the bottom of water. When humans soak themselves in pleasure, they're usually floating or standing in water.