Place the fruit, herbs, and sweetener if needed into a cocktail shaker and mash everything together with a muddler. If you don't have a shaker and muddler, you can also puree the fruit and herbs in a blender, but the juice will be cloudy.
Add the seltzer water and rum if desired, and stir well to combine
Cover the shaker with the strainer top and pour the mixture over 2 glasses filled with ice.
Garnish with additional herbs and melon balls as desired.