- Make a sugar syrup by mixing 2 cups of the sugar into 4 cups of the water, then bringing mixture to a boil. Cook until sugar is dissolved and syrup appears slightly thick, 5 minutes. Set aside to cool.
- Toss watermelon with remaining ½ cup sugar and salt. Let rest for 30 minutes.
- Using a food mill or food processor, puree watermelon. Mix with 2 cups of water and lemon juice. Churn in an ice cream maker according to instructions. Serve immediately or store in freezer.