A side effect of shooting underwater is that colors are filtered away by the water between your lens and the subject. A flash can help, but there will still be some color loss. You can bring back the full-color glory of sea life, though, by using the tools in Photoshop. In this movie, Deke McClelland shows you how.
To get the best results from this technique, you need to make sure you have enough color information in your image. But you can’t gauge this with the naked eye. Instead, go to the Channels panel in Photoshop and isolate the Red channel. If the channel appears too dark, this technique probably won’t work, but if your image detail is still clear, you can proceed.