Cut seedless watermelon into about 1 inch cubes. Please in a single layer on a baking sheet and freeze for 4 hours or until frozen through.
Add frozen watermelon, sweetened condensed milk, lime juice, and salt to a food processor or blender. Blend or process on high until smooth; 2-3 minutes.
Transfer the mixture to a large container. Seal and place in the freezer for additional 4 hours or until firm.