Blocking in the posture, proportions, and negative space body angles are the first steps in drawing a fish. You may see a parallel here to drawing birds, mammals, and everything else- it is the same system, just a different subject. Before you add any detail, block in these elements. If you add color with watercolor as in this demonstration, start with lighter values and work your way into the darks.
Click on the first image to start a step-by-step slide show.