Step 1: Select a ripe watermelon. Cut it into quarters, remove the rind and any visible seeds. Cut the flesh into approximately 1-inch cubes.
Step 2: Arrange the watermelon cubes in a single layer on a baking sheet lined with parchment paper, ensuring they do not touch each other.
Step 3: Place the baking sheet into the freezer for at least 3-4 hours, or until the watermelon cubes are completely solid. For best results, freeze overnight. Once frozen, transfer the cubes to an airtight freezer-safe container or bag for storage.
Step 4: When ready to serve, place the frozen watermelon cubes into a high-speed blender or food processor. Add the fresh lime juice.
Step 5: Blend on high speed, scraping down the sides of the blender as needed, until the mixture is smooth and creamy, resembling soft-serve ice cream or a thick slushie. If necessary, add 1-2 tablespoons of water to aid blending, but avoid adding too much to maintain a thick consistency.
Step 6: Taste the mixture and add optional sweetener (honey, maple syrup, or agave nectar) if desired, blending again briefly to combine. Serve immediately in chilled glasses or bowls for optimal enjoyment. If you prefer a firmer consistency, return the blended mixture to the freezer for an additional 30-60 minutes before serving.