Add water, sugar, and lemon juice to a saucepan and stir for about five minutes on low heat until the sugar dissolves.
Increase the heat to high and allow the mixture to boil for five minutes until the liquid thickens slightly. Remove from heat and cool for30 minutes.
Use a stick blender or food processor to puree the watermelon until smooth. Strain the fruit through a fine sieve into a bowl.
Stir the sugar syrup into the watermelon juice until combined, then refrigerate for two hours or until completely chilled.
Scoop the mix into an ice cream machine and churn according to the manufacturer's sorbet freezing instructions. When complete, transfer into an air-tight container and freeze until hard.
