Line a baking sheet or tray with parchment paper and spread the watermelon cubes in a single layer.
Freeze the watermelon until completely firm, about 3–4 hours or overnight.
Transfer the frozen watermelon to a high-powered blender or food processor.
Add the lime juice, sugar or honey, and salt (if using).
Blend until smooth and creamy, stopping to scrape down the sides as needed. If the mixture is too thick, add 1-2 tablespoons of water to help it blend.
Enjoy immediately as a soft-serve sorbet, or transfer to a freezer-safe container and freeze for 2-3 hours for a firmer, scoopable texture.
Let the sorbet sit at room temperature for 5-10 minutes before scooping and serving.