Cook the water, sugar and lemon juice together, stirring occasionally, until the sugar is dissolved.
Remove from the heat and chill.
Cut up the watermelon and remove all the seeds.Puree the watermelon in a blender or food processor.
Stir into the chilled syrup.
Freeze in an ice cream maker according to the manufacturer’s instructions.
Pack into an airtight container and store in the freezer until ready to serve.