Preparation - Mix the watermelon, lemon juice and sugar together. Refrigerate, covered, for 30 minutes. Pour the watermelon mixture into a blender or a food processor and process until smooth; set aside. Pour the water into a saucepan; sprinkle unflavored gelatin over top and let stand for 1 minute. Cook and stir over medium heat until the gelatin is dissolved. Remove from heat. Mix the gelatin mixture, the watermelon mixture and the evaporated skim milk together. Pour into an ice cream freezer (either a hand-cranked or an electric ice cream maker). Freeze according to the directions in the ice cream maker's instruction manual.