- Bring water and sugar to a boil in a saucepan, stirring until sugar dissolves.
- Cook for 3 minutes; remove from heat.
- Stir in punch concentrate, lemon juice, and pureed watermelon.
- Place in a plastic container and cover.
- Freeze at least 8 hours or until firm.
- To serve, thaw mixture slightly to form a slush.
- Spoon about 1 cup of slush in to a 12-ounce glass.
- Slowly pour in ½ cup of lemon-lime soda.