Place watermelon on a large rimmed baking sheet. Freeze until frozen, about 4 hours or overnight.Advertisement
Puree the frozen watermelon, condensed milk, lime juice and salt in a food processor, in batches if necessary, until smooth, 2 to 3 minutes. Transfer the mixture to a large sealable container. Cover and freeze until firm, about 4 hours.
To make ahead: Freeze for up to 1 week.
1 other carbohydrate, 1/2 fruit