I'd like to start a "series" here in the techniques and tips section for "Photoshop beginners." Please feel free to start new threads with tips of your own. Please start your Subject line with "Photoshop Tip - "
Underwater it is not always possible to get a perfect exposure and sometimes, a truly interesting or valueable shot may be under or overexposed. First tip - if you exposure is off, then err on the side of underexposure. You can always "bring up" this kind of shot, but an overexposed shots actually "loses" information. White pixels hold no info...
So, assuming you underexposed a shot, the best way to recover it is NOT to use the Image->Adjust->Brightness/Contrast, it is actually to use:
Image->Adjust->Curves
or
Image->Adjust->Levels
When using the latter, you will be looking at the Histogram. For an underexposed shot, you will notice that it is "scrunched" to one side (the black side). You can use the end slider to lighten the shot without losing any information. If the Histogram looks "flat" on one side, then slide the slider over until it just touches where the Histogram starts to come up.
To bring out detail in the shadows, try grabbing the slider in the middle and moving it back and forth. You'll be amazed what happens.
Try experimenting with different settings in Levels and Curves, as this quick posting only BARELY touches the surface.
Keep in mind that if you use "Lighten" or "Darken" it is just adding white pixels, or worse yet, DISCARDING black pixels. In essence, throwing away information from your shot.
Cheers
James