Ok, I have never told anyone this, but I also sometimes wake up and “see” like a waterfall effect over my vision. It only happens very rarely and only for less than a minute after I get woken from a deep sleep. It is not unlike a moving sensation or water moving over my eyes. I realized that it is probably fluid moving over my visual cortex and being interpreted as vision. Seems weird and I have nothing to back it up on, but it has happened multiple times over my lifetime and the sensation is exactly the same every time.