Oh boy, oh boy, oh boy! It’s the best of summer when you can have lemonade and watermelon at once. Be sure to taste the watermelon first. If it’s very sweet, reduce the sugar. Offer some refreshment to the planet and economy by going organic and local when you can.
Working in batches as necessary, purée watermelon and lemon juice in blender until smooth; transfer to a large container. Add water and sugar; stir until dissolved. Pour over ice in tall glasses, garnish with mint and thin slices of watermelon, if desired, and serve.