I've never done it with a reef/fish photo before but I've seen it done. There are a few ways to do it, you can use the lasso tool (or magnetic lasso tool in PS) and select the object you want in color, invert the selection and desaturate the selection. A better method and one I use when I do it, is to create a second layer, desatrurate it (there are several ways to get the B&W and some are better than others so research the different methods), add a layer mask (show all) to the desaturated layer, then use the paint brush and paint in black to erase the parts of the mask and allow the colored layer to show through. If you mess up, paint in white to fix it.