If you're trying to improve your underwater photography, one of the most useful things you can do is take photos on land. Doing so helps you get comfortable with your camera, with composition, technique, etc., and often leads to ideas for things to try underwater.
One thing I've been playing with recently is software to produce photographs that show a high dynamic range. Without going into too much detail, this basically means that your photograph displays a wider range of light values than normally possible.
This is accomplished by "combining" images taken at different exposure values, so you get detail in every part of the image, from the darkest to the brightest areas. This is really useful for times when there's harsh light and shadows, and your camera would have a difficult time capturing the entire range of light values in one frame.
Here's an example using Pasta as a model. The image on the left is an unadjusted, normally exposed image. The image on the right is an image created by running three images (bracketed at +1, 0, -1) through Photomatix software, using the tone mapping function.
The software allows me to create an image that more closely resembles what my eye saw. Our eyes are capable of seeing a much wider range of light than our CCDs are capable of capturing.
The obvious weak point is that the entire scene needs to be motionless in order to take and combine a number of images, so it's not suitable for action photography. I don't think I can use this underwater, but it's fun to play around with it anyway.
You can download the software from the Photomatix site for free to try, but the resulting images will have a Photomatix watermark applied until you purchase a registered copy.
A larger version of the dynamic range adjusted photo of Pasta is here.