In a small saucepan boil the water with the sugar, stirring, until the sugar is dissolved, simmer the mixture for 5 minutes, and remove the pan from the heat. Let the syrup cool to room temperature and chill it, covered, until it is cold. In a blender puree the mango with the pineapple juice, the cream of coconut, the lime juice, the zest, and the syrup until the mixture is smooth. Freeze the mixture in an ice-cream freezer according to the manufacturers instructions. Makes about 1 pint. Gourmet July 1993