To intensify a rainbow, I used a new one that I created in a separate file.
Copied and pasted a segment into the image, transformed scale to approximate the original.
Change layer blending mode to screen and use a layer mask to fade the ends.
Gaussian blur the rainbow and reduce layer opacity to suit.
Add a little Ocean Ripple to the rainbow below the horizon line
Finally, use the split slider in Layer Styles to break up the reflection in the water.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=26115817

original image by satelitegirl:

See next image for an updated rainbow template