Combine the limeade and the tofu in a blender, and blend till smooth. Add water and pulse to combine. Pour the mix into the freezer can of an ice cream freezer, freeze according to instructions. Spoon the mixture into a freezer container, cover and freeze 2 hours or until firm. Or you may put the mixture into a plastic container, freeze for about 2 hours, then process in a food processor until slushy, then freeze again until ready to eat. Take the mix out of the freezer and let it soften just a little before serving.