In a small saucepan combine the sugar with 1/2 cup water. Bring to a boil over medium heat, then remove from the heat and let cool.
In a blender add the watermelon, 2 tablespoons of the sugar syrup and the lemon juice and puree until smooth. Strain through fine mesh sieve into a bowl and discard the pulp.
Fill a pitcher with ice, add the watermelon mixture and stir until chilled.
Pour into chilled martini glasses and top with seltzer.
