Make a new layer and draw the outline of the puddle, filling it with red, add a bit of noise and a slight gaussian blur. Use the burn tool to put some highlights on the near edge of the puddle. Duplicate the original image on two layers, one above and one below the puddle layer, and cut away the top image to reveal the puddle beneath. Use the dodge tool to place some shadows from the body on the puddle, then reduce the opacity of the puddle layer slightly to give a little bit of transparency.

Annie