OK, I just want to give you a brief overview of what a histogram is and why they can be your best friend....
Basically, a histogram shows how much light is hitting your sensor, from light to dark.
This is an overly simplified and not entirely accurate way of thinking about a histogram, but if you think of it this way it will help you use it. If you look at a histogram, the left side is dark pixels and the right side is light pixels. The spikes indicate the amound of digital information in the picture resides in a certain area "lightness".
For example, if there is a huge spike on the left and it tapers off to the right, then it means that your picture is underexposed. If there is a huge spike on the right, and it tapers off to the left, that means that it is overexposed. If your information is pretty much spread between the two areas that means your picture is normally exposed.
What you REALLY REALLY REALLY want to avoid is a big spike on the right. Basically, the right means all white... that means you are 'blowing out' the image, and that it is overexposed. With a digital camera, once a pixel is recorded as white, there is no way to "recover" it to anything else... it is simply recorded as white, and that's that. On the other hand, if a pixel is recorded on the left side (the darker side) it can be amplified to show some color (even though this generates noise).
If in doubt, underexpose with a digital camera just a bit to keep your whites from blowing out. Blown out whites are quite typical with flash photography, and you want to avoid it.
I can't really tell you what a "good underwater histogram" looks like, because it will differ... but what you can do is look at the histograms of the pictures that you have taken that you really like (look at them in your photo editing software) and then compare them to what you are shooting.
After a bit, you will start to see what looks good.
Histograms help you avoid bad spikes... your screen on your camera is actually pretty good but it is still small and with fairly low resolution... and will appear bright underwater, not giving you a full sense of what you are seeing... the histogram will give you a 'math' idea of what the picture really contains.
Here's a good tutorial I found when I googled:
Image Histogram Explained
