Bring 6 cups water, sugar and salt to a boil in a medium saucepan over high heat, stirring occasionally. Remove saucepan from heat, add reserved lemons, cover, and let stand for 10 minutes.
Meanwhile, puree chopped watermelon in a blender or food processor with 1 cup water.
Pour syrup and watermelon puree through a strainer into a pitcher. Stir in lemon juice. (Lemonade can be made up to 1 day ahead.) Serve over ice.