Place the watermelon into a blender. Cover, and puree until smooth. Strain through a fine mesh sieve.
Bring sugar and ½ cup water to a boil in a saucepan over medium-high heat until sugar dissolves, about 5 minutes. Remove from heat. Stir in 3 cups of cold water and the lemon juice. Divide the ice into 12 glasses, and scoop 2 to 3 tablespoons of watermelon puree over the ice, then top with the lemonade. Gently stir before serving.