Making the base drink and adding the club soda later assures that all refills are properly fizzy.
Muddle mint leaves, agave nectar and lime juice in bottom of glass pitcher. Add rum. Adjust flavor to your liking.
In bottom of a tall glass, muddle watermelon. Add ice, and fill glass 2/3 full with mojito base. Top with club soda and garnish with a slice of watermelon and sprig of mint.