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"