If the watermelon is dusty, give it a quick wash and dry with a towel.
You will need to cut a flat surface on the watermelon so that it can stand on its own. To do this, cut off about 1/3 of the watermelon on a diagonal angle.
Place the watermelon, flat side down, onto your work surface (large platter or cutting board).
Cut out the mouth opening with a sharp knife.
Use melon baller to scoop out the watermelon. Leave 2-3" of the base intact. We learned the hard way that if you scoop out too much of the watermelon that the juices will leak out the bottom. Leaving 3" of watermelon will create a barrier.
Use a smaller utility knife or Exacto knife to carefully cut out teeth. To do this, score a shallow outline (about 1") around the mouth, only cutting through the green rind.
Then carve off the strip of green rind until you have a 1" white outline around the mouth (white rind of the watermelon). The white strip will be the teeth area.
Make triangle cuts all around the white outline area to create teeth in serrated pattern.
Remember the 1/3 of the watermelon that you cut off? Use that to cut out a dorsal fin and fins.
Attach fins with toothpicks. Draw an eye on with a permanent marker.