Copyright © 2005 Josh Audette

I think this turned out nicely. The biggest mistake is that the colour of some of the things outside the pool shows in their reflection (e.g. the flotation donut).

Technique:
duplicate image;
B&W-ify the duplicate;
copy-paste the B&W version into the original as new layer;
add mask (white for opacity);
use "hand-select regions" tool to select most of the pool and fill with black (transparent, so that the coloured version underneath shows);
manually paint the edges with the coloured version