Step 1: Make fish outline
Step 2: Fold it in half
Select both points from the line formed above and all points on the tail end side of this line. Put the 3D cursor on the line formed above and rotate all highlighted points 180 degrees along the Y axis (R, Y, 180). With the vertices from the tail end still selected, grab and move them all down the z axis (G, Z, # and direction of units).
Step 3: Insert eye
Make numerous faces for the front half of the fish, making sure to keep the eye hole open (select desired points, F).
Step 4: Extrude fish surfaces
Then, extrude each half of the fish along the z-axis (select appropriate faces on each side, E, Z, # units and direction for extrusion).