In a blender container place half of the watermelon, half of the rum, and half of the lemonade concentrate. Cover and blend until smooth. With blender running, add half of the ice cubes, one at a time, through the opening in the lid. Blend until slushy. (If the mixture becomes too thick, add a little water.) Pour into 2 chilled wine glasses. Repeat with remaining ingredients. Serve immediately. Makes 4 servings.