1) New Image: Background colour=Black
2) Decide what you want to make (eg, metal plaque, text, shape) the steps are all the same for whatever it is.
3) Make it. This one is just an elipse feather setting=2
4) Choose a colour. You can use just about any colour for this, however, the Saturation has to be 68 and the lightnes has to be 160.
5) Fill your selection with your chosen colour (Fill: Solid: Opacity=100)
6) Image: Plugin Filters: Filter Factory A: Pool Shadow...
1 - 70
2 - 70
3 - 70
4 - 70
5 - 70
6 - 70
Intesity - 130
Brightness - 160
7) Image: Cutout: Cutout Colour=Black: Blur=12: Horizontal and Vertical=-2
Play around with gradient settings for the colours.
This is the pool shadow default settings.