Is there a food-safe hydrophobic liquid with a substanially higher density than water?
Hello I know I went late to the party, but I have a solution to this. You all already have the right idea, but you need to put your ideas together.

Double layer your pool 

Whether it's gallium, water, oil, or some other liquid it will all work and be safe under glass.

Their are hydrophobic resins that you can treat the glass with, and if you double paned the area that you're waterfall would be under, it would not cause a health problem because there would be no contact.

For example if you had made window panes in your pool (let's just say in a t-shaped stretching halfway under your pool), and you coated the window panes with a hydrophobic resin, and you kept it under let's just say a temperature controlled environment, that would one solve your pool problem of being boring and two, keep your waterfall from fogging up.

So in short
1 Removable hydrophobic panels for cleaning
2 Heated/ chilled pool or temperature controlled environment
3 Heated / cooling pump for waterfall
4 Well designed lighting ( you'd be surprised what a little bit of light in the right spot can do)

I do not know if it's available to the public yet but there is a coolant used for computers that is one consumable and two low maintenance called Coriant. It has a lighter density than water, if you're  panes are hydrophobate this would not present a problem.

Colored mineral oil or antifreeze would also work here.

If you use antifreeze and we're creative with its use, it may assist in some other mechanical uses as well as being a display piece.

Please note that any hydrophobic material or substance is going to be toxic to breathe or have extended contact with.

This should not be a problem if it is under a air / water tight encasing.

I apologize that I'm so late in responding to this but I hope someone reads this and finds it useful thank you for your time.