How to Make It
Combine watermelon juice, chilled rum, chilled simple syrup, lime juice, and bitters in a 2-quart pitcher; stir to combine. If not serving immediately, seal well with plastic wrap, and refrigerate up to 2 hours.
To serve, stir watermelon mixture well. Fill pitcher with ice, and stir gently until outside of pitcher is cool. Pour into rocks glasses filled with ice, and garnish each glass with a lime wheel, mint sprig, and small watermelon slice.