Open the image and use your eyedropper tool to select the lt. blue. Then Image/Adjustments/Hue and Saturation. Go to the menu that says Master at the top, and drag down to "Blues" (we can come back and do "Cyans" if you don't get it all) Notice the "pointers" (for lack of a better term) surround the blue in the color bars; drag the Brightness slider to the right until the blue disappears. You may want to adjust the pointers to get just the blue you're looking for. Check and uncheck the Preview box to see what's happening. This is where you want to go to do this, and be careful, the water is deep here (I know it's over my head)
Steve