Chop the watermelon. In a cocktail shaker, muddle the mint leaves. Add the watermelon cubes and muddle again until they are turned into juice.
Add the rum, lime juice and simple syrup and fill the cocktail shaker with ice. Shake until cold.
Add ice to a glass, and strain in the drink. Top off the glass with soda water. Garnish with additional mint leaves and watermelon wedges.
Pitcher variation (8 servings): Muddle 4 cups watermelon into a juice (or use 2 cups watermelon juice). Add to this Mojito Pitcher recipe, and garnish with additional watermelon slices.
Notes
*To convert to tablespoons, 1 ounce = 2 tablespoons