In a medium saucepan over medium heat, combine the water, sugar and lemon juice. Cook while stirring until the sugar is completely dissolved. Cool completely.
Carefully pour the liquid into the blender. Place the watermelon over the liquid. Cover the blender and process until the mix is completely smooth. Transfer to bowl of electric ice cream maker. Make sorbet according to manufacturer’s directions.
If you don’t have an ice cream machine, pour mixture onto a parchment-lined cookie sheet and freeze until set. Break it up and blend in food processor or blender until smooth. Freeze and blend again for an ultra smooth consistency.