How to Make It
In a small saucepan, bring the water, sugar and ginger to a boil. Simmer over moderate heat for 5 minutes. Strain the syrup into a large pitcher, pressing hard on the ginger. Let cool to room temperature.
Stir in the lime juice, rum and the 2 cups of ice. Strain into ice-filled glasses and garnish with lime slices and cilantro.