Last Sunday, Sydney (where I live) was the hottest place on the planet. We made this delicious Watermelon Sorbet and enjoyed it without a shred of guilt!
Watermelon, peeled, cut into chunks and frozen
Place frozen watermelon in a Vitamix (or other high speed blender)
Blend until a smooth, sorbet consistency is achieved - you may need to use the tamper accessory
If it still doesn't blend well add in a little non-frozen watermelon or a tiny bit of water/juice.