Combine mint leaves and lime juice in the bottom of a tall glass. Lightly muddle. Fill glass with ice. Add cranberry simple syrup and light rum. Top with soda water. Lightly stir with bar spoon. Garish with fresh cranberries and limes (optional).
Cranberry Simple Syrup: In a saucepan, add 1 cup water, 1 cup sugar and 8 oz of fresh cranberries. Bring to a light simmer over medium-high heat until sugar is dissolved and cranberries start to burst (around 10 minutes). Remove from heat and strain out cranberries.