Combine the sugar and water in a small saucepan over medium heat. Stir until the sugar is completely dissolved, about 2–3 minutes. Remove from heat and let cool to room temperature. (Tip: You can speed this up by placing the syrup in the fridge for 10 minutes.)

Add the cubed watermelon to a blender and blend on high until completely smooth, about 30–45 seconds. You should have approximately 4 cups of liquid puree.

Add the cooled simple syrup, fresh lime juice, lime zest, sea salt, and mint leaves (if using) to the blender. Blend briefly for another 15–20 seconds until fully combined and smooth.

Taste the mixture. It should be bright, slightly sweet, and very refreshing. Adjust with more lime juice for tartness or more syrup for sweetness as needed. Remember that freezing will slightly dull the sweetness, so err on the slightly sweeter side.

Pour the blended watermelon mixture into a 9x13 inch baking dish or other wide, shallow freezer-safe pan. Spread evenly.

Place in the freezer uncovered for 45 minutes, until the edges begin to freeze and the center is still slushy.

Using a sturdy fork, scrape the frozen edges toward the center, breaking up any ice crystals. Stir and spread back out into an even layer.

Return to the freezer and repeat the scraping process every 30–45 minutes for a total of 3–4 hours, until the entire mixture has been transformed into light, fluffy, icy crystals with no remaining liquid.

Once fully frozen and flaky throughout, cover the pan tightly with plastic wrap or transfer to an airtight freezer container.

To serve, fluff the granita with a fork and scoop into chilled glasses or bowls. Garnish with fresh mint leaves, a slice of lime, and a pinch of flaky sea salt if desired. Serve immediately.