Couple of things will help
1. Eliminate or try to reduce Backscatter, it’s the result of lighting an underwater scene too straight-on in water that’s not crystal-clear. Backscatter means you’ll get an image in which the background is filled with hundreds of speckled dots. The focus should be on the grouper.
2. Contrast is so important. When taking the pic, the background and subject blend can blend together, featuring little contrast. A simple example of a badly contrasted image would be when you shoot the grouper in front of a reef similar in color. The lack of different tones and colors means it will all meld together, creating an image that doesn’t grab your attention or focus your eye. Unless you’re trying to show off a creature’s camouflage, it’s best to reject images that lack contrast.
3. There is a general rule in photography that if you’re shooting a photo of a fish or some other animal, including a diver, you must either capture the whole body in the shot or crop out at least 40 percent of them. A good example is an image featuring a diver whose hand is cut off in the image or whose fins are mostly cropped. These images cause the viewer’s eye to jump off the page or screen, thus ruining the sense of immersion the photo might create. Therefore, any images of people or your grouper with small missing parts from the frame are easy targets for your computer’s trash can.