Greetings 'a',
Well, first of all lets look at the reasons why the image fell below your initial expectations...
Digital cameras or film cameras come to that would have had problem with this particular set of circumstances.
Shooting a minority dark subject against an overwhelmingly bright background without the aid of artificial light or an advanced spot metering system will end up similarly - silhouetted.
Lowend digital sytems hate high contrasts and the internal strobe is the only way of rectifying this to any degree, but you have to be close - within a couple of feet (underwater) of the subject.
Choose the highest quality picture setting on the cameras menu before you enter the water (SHQ). Shots available governed by the smart media memory but on 1.3 mp: 32/64 mb is usually sufficient for a dive.
Set the cameras close-up facility and hold the unit (in a moray scenario) at arms or elbow length and slowly depress the shutter button with your thumb whilst remaining still and looking into the screen. Review if possible and reshoot if necessary. Focus yourself, not just the camera.
Try to get on the same level as your subject or slightly below it and allow it to become used to you - [i appreciate diveguides want to whizz you round but, if you make them aware what you're upto they will hang on] - Then compose the shot you think most asthetic, or a shot members of your club would like to see, depicting the character and habitat of that particular beasty.
Topside: Save your images at maximum size and resolution onto your storage device and only reduce copies. Larger files contain more information to enable post manipulation, and obviously boost its print potential. The corrected & stored images then be reduced as copies for email etc.
The image as it is would be very difficult to work on and would take a lot of time in PhotoShop but if you have saved a larger file why not try - There is a lot of information in the moray itself. First you will have to isolate the moray with the marquee tools (save selections - at least 2 seperate cuts). Then you can work on reducing the brightness of the background and bring up the contrast in the moray and adjust the colours, hues & saturation. The image can be cropped by 30% to provide a better composition.
Each species of marine life lives in its own little world be it alone like or shared, it's nice to capture these characteristics as much as you can.
Hope that helps?
Plenty of great stuff in our seas and lakes too!