1.Combine water and sugar in a saucepan and bring to the boil.
Remove from heat and allow to cool completely.
2.In a food processor or blender puree watermelon flesh and strawberries.
3.Mix puree into sugar mixture and freeze in a stainless steel bowl.
Whisk from time to time during freezing to give a smooth even texture.
To serve, spoon into elegant glasses and at the last minute pouratablespoon of champagne into each glass.