YES, that was the issue! THANK YOU!
Wow this is not good because I wanted the cave to be pitch black. But I semi solved this by making the cell's "light color" BLACK (well a very dark gray anyway)!
Then I can illuminate it the way I want to with my own placed lights.
I've read that waterfalls may become pretty much invisible depending on the lighting.
Try temporarily adding a lot of light and see if it shows up.
and
Increase the ambient lighting in your cell. Values around 100, 100, 100 will make it visible.
Waterfalls are made for exterior locations so they do not show up well inside with it being so dark.