I found a website with a personal guide to diving and staying in southern Baja California that's quite useful:
It included this photo of a ray:
I copied this pic into Photoshop Elements 11, applied a little color correction voodoo, and got this:
So the next time you see a green-tinted underwater pic, you should realize that it may be possible to color-correct it pretty easily (if the subject isn't too deep).
For example, with another photo from the same website:
...I was able to remove the color cast but couldn't restore the original colors:
It's still an improvement, but I want to set your expectations realistically.