Muddle lime, rose petals with rose syrup and rose water in a mixing glass. Add rum and shake in a cocktail shaker with ice. Strain the mixture into a highball glass over ice and top off with champagne. (Rose water is a distilled rose flavored water, used mainly to add aroma to the cocktail. It contains zero carbohydrate and zero gram of calorie.) Rose water is often used in traditional Mexican dishes that use dry roses for flavoring.