This was in the most recent Runners World.
Cube 8-10 cups of watermelon (discard seeds).
Puree in blender until you have 4 cups.
Mix with 2 Tbsp lime or lemon juice and 1 cup simple syrup (boil equal parts water and sugar until sugar dissolves; then let syrup cool).
Chill in an ice-cream maker, or leave in your freezer until slushy.