Okies, this is what i came up with. Not sure if this looks like natural blue water but its passable. I did this in a hurry so i'm sure theres still some parts of the photo that look slightly brown, but i can do a more professional job later if you like. =)
What i did :
Adjusted levels, adjusted color balance-gave it more blue and more green, more red.
then added a layer, set it to color, opacity 26, fill 86. and filled in a light greeny blue color.
erased where her head and dress it so there aren't blue in those areas
created another layer, set mode to color, opacity 45 and added a darker blue over top with the paint brush with that opacity of 59.
hope i helped !! =)