Cut watermelon into roughly 1-inch cubes and spread in a single layer on a parchment-lined baking sheet. Freeze for a minimum of 4 hours or overnight until completely solid.

Combine 1/4 cup granulated sugar and 1/4 cup water in a small saucepan over medium heat. Stir until the sugar fully dissolves, about 2–3 minutes. Remove from heat and let cool completely before using. You can make this ahead and refrigerate it.

Add the frozen watermelon cubes to a high-powered blender. Pour in the cooled simple syrup, fresh lime juice, kosher salt, and vodka if using.

Blend on high until completely smooth, stopping to scrape down the sides as needed. The mixture should be vibrantly colored, velvety, and fully pureed with no visible chunks. Taste and adjust — add more lime for tartness or a splash more simple syrup for sweetness.

Pour the blended mixture into a loaf pan or freezer-safe airtight container. Press a sheet of plastic wrap directly onto the surface of the sorbet to prevent ice crystals.

Freeze for 1 hour. Remove and stir vigorously with a fork, scraping the frozen edges into the center. Repeat this every 45 minutes for 2–3 more rounds to build a smooth, scoopable texture.

After the final stir, smooth the surface, replace the plastic wrap, cover with a lid, and freeze for a final 2–3 hours until fully set and scoopable.

Remove the sorbet from the freezer and let sit at room temperature for 5 minutes to soften slightly. Scoop into chilled bowls or glasses, garnish with fresh mint and a lime wheel, and serve immediately.