Bring the water to a boil, remove from the heat, and add the sorrel and ginger. Let steep about 45 minutes, strain.
Add ice cubes to a shaker. For 2 servings, add to the shaker about 2 or 3 ounces strained juice, 2 ounces simple syrup, a splash of lime juice, and a jigger of rum. Shake well and pour.