Step 1: Prepare the watermelon. Cut the seedless watermelon into approximately 1-inch cubes. If using a seeded watermelon, remove all seeds before cubing.
Step 2: Blend the mixture. Place the watermelon cubes, fresh lime juice, simple syrup (if using), and an optional pinch of salt into a high-speed blender. Blend until the mixture is completely smooth and liquid. If you prefer a very smooth sorbet and used a seeded watermelon, you can strain the mixture through a fine-mesh sieve at this point to remove any pulp or tiny seeds.
Step 3: Chill the base. Pour the watermelon mixture into a shallow bowl or container, cover, and refrigerate for at least 2-4 hours, or until very cold. This pre-chilling step is crucial for optimal sorbet texture and prevents large ice crystals from forming.
Step 4: Churn the sorbet. Transfer the chilled watermelon base to your ice cream maker. Churn according to the manufacturer's instructions, typically 20-25 minutes, until the sorbet reaches a soft-serve consistency.
Step 5: Freeze to firm. Spoon the soft sorbet into an airtight freezer-safe container. Cover and freeze for an additional 2-4 hours, or until it reaches your desired firm, scoopable consistency. For best results, allow the sorbet to sit at room temperature for 5-10 minutes before serving to soften slightly.