Ok. Below is the original, straight out of my camera and the processed image.
My technique? It varies from image to image, but here's what I did --
I created a "Selection" of Damaris, then inverted it to select just the water, then used "Image-Adjustments-Replace Color" to select the brown water and turn it blue (I repeated it several times to get all the shades of the water to the correct shades of blue). Then I loaded the selection of Damaris again and bumped up the contrast. I used the Clone tool to clean up her face a bit. As a final step, I applied a subtle vignette using Levels to give it a deeper blue look. Image was then flattened and unsharpenes at 100-3-0 setting. Voila!
Oh ... and yes, right after taking this shot, she rolled over onto her back and her bikini bottoms filled up with water and slid to around her knees ... while I was repositioning myself on a rock. :-(