Add the water and sugar to a saucepan and stir for about 5 minutes on a low heat until sugar dissolves. Increase heat and bring to the boil. Continue to boil until liquid thickens a little the remove from heat and allow to cool for 30 minutes.
Place watermelon in a blender and blend until pureed. Strain watermelon through a fine sieve into a bowl.
Add sugar syrup to watermelon juice and stir until well combined.
Add to ice cream machine and churn as per manufacturers sorbet freezing instructions. When complete, scoop into an air-tight container and freeze until completely hardened.