How to Make It
Combine 1 cup water and sugar in a small saucepan; bring to a boil. Reduce heat, and simmer for 5 minutes or until sugar dissolves. Remove from heat. Stir in basil; chill 1 hour. Strain sugar mixture through a sieve over a bowl, and discard basil.
Place watermelon in a blender; process until smooth. Pour watermelon puree through a fine sieve into a large bowl, reserving liquid; discard solids. Combine watermelon liquid, sugar mixture, and juice in a pitcher; chill.