This is what I created for a snack at my son's backyard summer celebration a couple of weeks ago. It wasn't terribly difficult - much the same as carving a pumpkin at Halloween. All I did was cut a large wedge from the side, then I scooped it all out with a melon-baller. Then I cut the triangle teeth and carefully cut away the rind so that they would be white. The fin is made from the rind of the wedge I cut out at the start, and I attached it with a couple of toothpicks. The eye is just a grape on a toothpick that has been pressed into the rind. Ta-da! Watermelon shark!
You could just fill it back up with the balled watermelon, but I wanted a variety of fruit, so ours is filled with honeydew, pineapple, grapes, strawberries, plus the original watermelon.
The kids LOVED it! They thought it was so cool! They all ate the fruit really well and I got to feel good about serving a healthy snack at the party that they were all excited about.