On a cutting board, cut off the bottom 1/4-1/3 (depending on size) of the watermelon on an angle. Stand the watermelon up on the cut surface, making sure it is steady. Find the midpoint of the watermelon at the top and draw a line marking where the top of the mouth will be. Mark another line below it, to make the bottom of the mouth opening. Using a paring knife, cut along the lines and pull out the mouth piece. Reserve.
Scoop out all flesh out of the watermelon, leaving a gap at the bottom, so that the fruit salad doesn’t fall out.
One inch above the top and bottom of the mouth, score a line and scrape off all green skin to show the white rind. Carve small triangles into the top and bottom of the mouth to form shark teeth.