Sometimes you are going to get what you get with the DR of your system. I am assuming that you are shooting TTL with the SB80DX, is that correct? What I see in the picture is very hot, white spots and lots of dark eel. First, you may need to play with strobe position. Try to aim the strobe at different angles to the eel, preferably low and to the front, as Alex suggests. I have found that strobe position can make a HUGE difference when shooting TTL. Over shooting the subject with strobe light will expose for the wrong areas such as the sand, causing the camera to shut off early because it exposed the light sand, not the dark eel. Thus you end up with a dark eel and properly exposed sand/reef. The only other way to correct this is don't use TTL. No matter how much light you have, the camera will shut off based on the reflected light. More light means the camera will shut off sooner.
However, if you shoot manual, you can expose for the eel much easier and with far greater control of your lighting. Strobe position is critical as well so you don't blow out the lighter highlights such as the background. Sometimes, depending on the position of the subject to the camera, it is not possible to put your strobes in a optimal position so you may have to live with the hot spots. But in manual you can adjust your f-stop or your shutter speed to properly expose the eel.
With your existing picture, make some basic tweaks in raw exposure, WB, color temp and some slight contrast then pull it into PS. The very first step I do on pics like this is to perform a shadow/highlights action. That should bring out the eel. Drag the "shadows amount" slider to the right until you get an acceptable exposure. Don't over due it because you will simply ruin the picture and get unwanted noise. Next, drag the highlights amount slider to the right and watch what happens. Select a level that pleases you and click okay!!
Here is your picture with some shadow/highlight adjustment. Then I did a little desaturate on the eel and some dodging. It's still a very solid idea to get the exposure as accurate in the camera as possible to maximize your image quality after post processing. Many potentially great pictures are tweaked way too much in PS and it saimply ruins the quality of the image.
Joe