1 (7- to 8-pound) seedless watermelon, rind removed, cut into large chunks (about 12 cups)
1-1/2 cups fresh lemon juice (from about 6 large lemons)
Roughly chop half the basil leaves. (You should have about 1/4 cup.) In small saucepot, heat water and sugar over medium heat 3 to 5 minutes or until sugar dissolves, stirring frequently. Remove saucepot from heat; stir in lemon zest and chopped basil. Let stand 10 minutes.
Meanwhile, in blender or food processor with knife blade attached, in batches if necessary, purée watermelon until smooth. Into large pitcher, strain watermelon purée through fine-mesh strainer; discard solids. Into same pitcher, strain sugar mixture; discard solids.
Stir in lemon juice. Refrigerate at least 2 hours or up to 3 days. Stir in remaining basil leaves just before serving. Makes about 8 cups.