I cropped it a little differently. The most noisy parts were the dark areas so I actually made them darker and thus more uniform. Then I used the color selector tool, selected the orange areas of the fish/fins and increased color saturation, also lightened up the orange where I could. Finally I selected all of the green area and used a gaussian blur filter with a little brightening. As you learn to control exposure more, your pictures will contain so much detail that your post-processing options will expand accordingly. A poorly exposed photo has a lot more junk in it than actual 'data' so post-processing options are slim to none really.