First, make a simple syrup by placing the sugar in one cup of water in a small saucepan on medium heat. Cook it while stirring constantly until it dissolves. Let the syrup come to a boil for five minutes, then remove it from the heat and let it cool completely.
In the meantime, blend the watermelon until it’s smooth. Muddle the limes in a pitcher, then add the pureed watermelon, soda water, lime juice, and simple syrup. Mix well, then add the mint.
To make sugar to rim the glass, mix the two reserved tablespoons with the lime zest on a small plate. Place 1/4 cup water in a separate bowl, then dip the rims of your serving glasses in the water, then in the sugar.
To serve, pour the watermelon mixture over ice cubes.