It's very weird, because your Quick Mask button is presses, but the title bar doesn't say you're in QM mode.
That must have something to do with it, though.
The other thing we need to test is if you can Alt-click on the mask of another layer (one that's already been painted on) to see if it show the black-and-white view that it's supposed to.
May I see the whole screenshot, so I can see the little border around the mask thumbnail?
Not saying I don't trust you ... um, actually, yes, that is what I'm saying ...
Try this ... a Channel Mixer adjustment layer, with these values:
Red 0, +95, 0
Green 0, +110, 0
Blue 0, +45, 0
Then invert the mask to hide the adjustment, then paint on gently with a low-opacity white brush.
Well, you'll need photos of water from other pools, I figure. Either look through your own library of photos, or go to some stock websites to see if they have suitable ones.
Sorry, I need a better explanation. Fill in these blanks:
"My screen is __________ (colder/warmer) and ________________ (brighter/darker) than my prints"