Heat the soup and water in a 1-quart saucepan over medium heat until the mixture is hot and bubbling, stirring occasionally. Spread the butter on the bread slices. Place 4 bread slices, butter-side down, into a 12-inch skillet. Top with the cheese slices and remaining bread slices, butter-side up. Cook over medium heat until the sandwiches are lightly browned on both sides and the cheese is melted.