Dice watermelon and remove seeds if not using seedless fruit.
In blender, combine watermelon, coconut milk, lemon juice (optional) and coconut syrup (if using for additional sweetness). Blend until smooth.
Pour watermelon coconut puree into a baking dish, cover with plastic wrap and freeze until solid (about 4 hours). I had to leave home so my puree ended up sitting in the freezer overnight. If that happens to you too, leave about 1/2 hrs at room temperature, before proceeding to the next step.
With a sharp knife, cut frozen puree into cubes.
Place cubes in a blender/food processor and process until the texture is silky smooth. If your blender/food processor is not powerful enough, you might want to add the cubes in small batches. If too frozen, add a few drops of coconut milk.