Combine sugar, water, and lemon juice in a medium saucepan over medium heat. Stir constantly until the sugar completely dissolves, about 5 minutes—you'll know it's ready when you no longer feel any grittiness when you run a spoon across the bottom of the pan. Remove from heat and stir in the corn syrup and salt, which will add smoothness and prevent the sorbet from becoming too icy. Transfer the mixture to a bowl and refrigerate for at least 30 minutes while you prepare the watermelon. I find that chilling the syrup first makes it blend more smoothly with the frozen fruit and creates a better final texture.

While the syrup chills, add the watermelon cubes to a blender in batches if needed and blend until completely smooth with no visible chunks or pulp. Strain the blended watermelon through a fine-mesh sieve into a bowl, pressing gently with the back of a spoon to extract as much liquid as possible while leaving behind any remaining fibrous bits. This step creates a silky texture in your final sorbet rather than a grainy one.

Pour the chilled simple syrup from Step 1 into the strained watermelon puree from Step 2 and stir until well combined. Transfer the mixture to your ice cream maker and churn according to the manufacturer's instructions, typically 20 minutes, until it reaches a soft-serve consistency. The sorbet should be smooth and creamy when it comes out of the machine—I like to serve it immediately for the best texture, though you can transfer it to a freezer-safe container for storage if needed.

Scoop the freshly churned sorbet into chilled serving bowls or glasses and serve immediately. The bright, refreshing flavor is best enjoyed while the texture is still light and creamy.