Combine rum, watermelon juice, lime juice, and simple syrup in a resealable container and stir well to mix. If not serving right away, cover and refrigerate for up to 2 hours.
Carefully pour tonic water into a 2-1/2-quart pitcher. Stir watermelon mix well, then add to the pitcher as well. Give it a brief, gentle stir and pour into ice-filled collins glasses. Garnish each glass with a cucumber wheel.