Place watermelon cubes and raspberries in blender; process until smooth. Pour juice through a sieve; discard pulp.
Combine sugar and water in small saucepan. Heat, stirring constantly, just until sugar dissolves. Set syrup aside and allow to cool slightly.
Stir syrup and lemon juice into watermelon juice mixture. Pour into freezer container, cover and freeze until firm.
Using ice cream scoop, spoon mixture into tall glasses. Pour 3 to 4 tablespoons strawberry-flavored water over frozen juice in each glass. Serve immediately.
VARIATION: To serve as sorbet, freeze juice mixture in 8 inch square dish. Omit strawberry-flavored carbonated water. Using a small scoop, shape frozen juice mixture into balls and arrange in individual serving dishes.