Spread the watermelon cubes onto a lined baking tray in a single layer.
Freeze for 3 to 4 hours, or until fully firm.
About 30 minutes before you are ready to blend the frozen watermelon, add the sugar and ¼ cup water to a small saucepan over medium heat.
Stir for about 5 minutes, until the sugar fully dissolves and the liquid looks clear.
Pour the simple syrup into a separate container to cool for 30 minutes.
Add the frozen watermelon, simple syrup, juice of 2 limes, and salt to a food processor.
Blend for about 5-8 minutes, until the sorbet looks smooth and evenly pink with no large fruit chunks left. It should look like a soft scoop texture at this stage.
Transfer the mixture to a 9x13-inch baking dish
Cover and freeze the sorbet for 2 to 3 hours.
Once set, let the sorbet sit at room temperature for 5 to 10 minutes before scooping so it softens slightly.
Garnish with lime zest and optional mint before serving.