Basic underwater image tweaking tips.
These tips relate to Photoshop Elements 2.0 but the same adjustments will be available in most image programs.


A note of caution before you dive in to manipulating your images:
Always use "Save As" (rather than 'Save') and change the name of your image to prevent overwriting your original.
I tend to add "a" to the filename to indicate "adjusted". This means image234.jpg once adjusted is saved as image234a.jpg
Or make a copy of your original and work on the copy.

The most common tweaks usually required are:
Adjusting underexposed images
Adjusting colour casts
Removing backscatter
Cropping
Resizing and compressing for email or web use

Adjusting exposure

Open your image in Photoshop
Go to Enhance>Adjust Brightness/Contrast

Now you have a choice of Brightness/Contrast or Levels.
Brightness/Contrast is easy to understand, you just fiddle with the sliders until the image looks OK but using Levels is a much better way to do it.

Select Levels and a graph or histogram will appear. Don't be scared off, it's not complicated. 

This graph shows how many pixels in the image are at each brightness or tone level. Black is on the left and white on the right. In this example there are lots of darker pixels and less lighter pixels, typical of an underexposed image.

See the three small triangles along the bottom of the graph. You use these sliding triangles to tell Photoshop where you want black or the mid tones or white to end up. 

OK, here's the magic part.
Grab the right triangle, drag it left towards the graph and watch what happens to the image.
Next you can grab the middle triangle to change the brightness of the mid tones and pull the left triangle to the right a bit if you need to darken the shadows as well.  An advanced trick is to hold down the Alt key while dragging and you will see exactly which areas are turning pure black or pure white.

Click OK when you're happy with the result.

Adjusting Colour casts

You can also use the Levels to adjust overall colour. Open Levels again.
Click in the Channel box and you will see Red, Green and Blue levels are available individually.
Select Green and a graph for just the green pixels will appear. Make adjustments by moving the middle triangle and repeat for Red and Blue if required.

Here's another method. Go to Enhance>Adjust Colour>Colour Cast.
You then just click on part of the image that should be a neutral colour like White, Grey or Black and the overall colour will be magically adjusted.
You may need to click on a few different spots to get the best result.

You can also try Enhance>Auto Levels but I find this over-does it with most underwater images.

Removing Backscatter with the Clone tool

Backscatter can totally ruin an underwater image. There's not much you can do about images with heaps of backscatter but small amounts can be easily cleaned up using the Clone Tool.
This tool acts like a brush that picks up one small patch of the image and paints it onto another part.
The Clone tool looks like a little rubber stamp .
Once you have selected it look along the top toolbar and select a soft brush (streak with blurred edges), Size about 50, Mode Normal, Opacity 100% and Aligned.

Now move the cursor over the image. You can quickly adjust the Brush size to be just larger than the backscatter by using the ] and [ keys on the keyboard.

Move to a clean part of the image just beside a speck of backscatter and alt-click to pick up a patch of image, then click on the backscatter speck to paint over it.
You can now go right over the image painting out all the backscatter. The Clone Stamp will continue to pick up a patch of image the same distance away from the stamp until you redo the alt-click.

Cropping

Your image may be improved by chopping off part of the space around the subject
Select the Crop tool.
Click and hold the mouse on the image and drag the cursor across to select the area you want to keep. You can move this selection box around once you have drawn it. Hit Enter to complete the crop or click on the crop tool again to cancel it.

Here's a neat trick. If you want to resize the image at the same time you can type the desired pixel size into the width and height boxes along the top tool bar. The image will be cropped and resized in one action.

The rectangular selection tool gives you even more cropping options.
You draw a selection box the same way as the crop tool but after the selection is made you go to Image>Crop to complete the crop. The other options are found in the Style box on the top toolbar. You can choose Normal, Fixed aspect ratio or Fixed size.

Resizing

The images straight out of your camera will be too large for email and websites so you must learn how to resize and compress images.

Go to Image>Resize>Image Size. Make sure Resample Image is ticked and type in the number of pixels you want.

Here are some recommended image sizes in pixels:
Email/web - 640 x 480
DVD slideshow - 800 x 600
Powerpoint - 1024 x 768
Print - don't resize

After resizing most images will benefit from a little sharpening. Go to Filter>Sharpen>Sharpen

Saving images

What file format should you use to save your images?

For email, websites, DVD slideshows and Powerpoint, only save as jpeg.
Jpegs are compressed when saved to much smaller file sizes. The down side is that some quality is lost in the process but this is often not noticable. When you choose Save As jpeg the amount of compression should also be selected. You need to play with this to work out how much compression is OK without degrading the image too much. The "Save For Web" option in Photoshop gives you the most control over quality vs file size.
Images that you wish to work on repeatedly to achieve the final result should be saved as PSD (Photoshop's own format) or TIFF to avoid progressively degrading the quality.

Some cameras can produce RAW images which are uncompressed and unadjusted by the camera. RAW images need more advanced treatment than is covered in this discussion.