Prep time does not include chill time.

1/2 cup granulated sugar

1/2 cup water

6 cups cubed watermelon

3 tablespoons lemon juice

1 cup seltzer water

In a small saucepan, bring the sugar and water to a boil, whisking often. Boil for 1 minute then remove from the heat.
In a blender, blend together watermelon, sugar water, and lemon juice. Pour through a fine mesh strainer into a large bowl, discarding any solids and seeds left behind. Refrigerate until cold, about 1 hour.
Reserve 1 cup watermelon mixture for the soda. Churn the remaining mixture according to your ice cream maker's instructions then freeze until firm, preferably overnight.
Right before making the floats, stir seltzer into the reserved 1 cup watermelon mixture. Scoop sorbet into serving glasses then pour soda over top. Serve immediately.