To make the syrup, bring the water and sweetener to a boil over medium heat, then simmer it for 15 minutes on low heat.
Put the cantaloupe in a blender and blend until smooth.
In a large bowl, mix the syrup and the melon until well combined. Add the lemon juice. You can adjust the taste according to your taste. Remember that when it freezes, it will lose a fraction of its taste so make the cantaloupe mixture a bit strong.
When the cantaloupe mixture cools down, transfer it to an ice cream maker and follow the instructions of your machine.
If you are not using an ice cream machine, transfer the mixture to a plastic or metal container with a cover and place it in the freezer. Take it out every half an hour to mix it until it arrives to the perfect consistency of a granita.
If the granita hardens too much, break it into medium to big pieces and transfer it to a mixer to break it apart.