For a no-sugar version, use extra-sweet watermelon and omit the sweetener. For Watermelon Lemonade Soda, replace 1 cup of water with sparkling water just before serving. For a concentrate version, use 8 cups watermelon, 1 cup lemon juice, sweetener to taste, and only 1/2 cup water. Store covered in the refrigerator for up to 2 days and stir before serving, as fresh watermelon juice naturally separates.