Grilling watermelon adds a smoky note to the fruit and gives it a slightly meaty texture that matches well with shrimp. Cooking takes only a couple of minutes, so have both sets of skewers ready before you start. You'll need 12 flat metal skewers (12 in. long) or 18 skewers (8 in. long)--or use bamboo skewers of the same size, soaked in water for 30 minutes.