Remove the rind of your watermelon and chop it up roughly. Add it with the mint leaves and lime juice to a blender. Blend until it is evenly combined. If your blender is not big enough, then do this in stages.
Strain the juice through a mesh sieve and discard the solids.
Pour the juice and agave syrup into an ice cream machine and process it.
Transfer into a freezer friendly dish and freeze for another 1-3 hours before serving if you want a scoopable sorbet.
You can adjust the mint according to your taste, the more you add, the more refreshing it will be.If you don't have an ice cream machine then you can also pour the fruit juice into a freezer friendly dish and place it as it is in the freezer. Set a timer and give it a good stir every 30 minutes until it is frozen.