Colour Balance
The Colour Balance command changes the overall mixture of colours in an image for generalised colour correction and is very valuable when correcting underwater images. Select Image menu > Adjustments submenu > Colour Balance. In the Colour Balance dialog box you can select the tonal range in which you want to focus the changes (i.e. shadows, midtones or highlights). You should check the Preserve Luminosity option to maintain the luminosity values in the image while changing the colour.
There are three sliders with the following relationships: Cyan/Red, Magenta/Green and Yellow/Blue. Those of you, who are familiar with colour theories, will recognise the above relationships. All you have to do is drag a slider toward a colour that you want to increase in the image; drag a slider away from a colour that you want to decrease in the image, while the tonal range you want to affect is checked.