ready in 4 h. 20 min.
Put the watermelon flesh, salt and lime juice in a food processor or blender and blend until combined.
Push the mixture through a fine sieve into a freezerproof container. Stir in the sugar until dissolved.
Freeze for 1 hour, then mix the granita with a fork. Return to the freezer and freeze for a further hour. Continue this twice more until the mixture is completely frozen. Once frozen, scrape with a fork to form fluffy ice and spoon into small glasses.
Decorate with mint leaves.