Add ⅓ of the cubed watermelon (330 g) to a small saucepan. Simmer over medium-low heat for 10–12 minutes, stirring occasionally, until reduced by half (about 160 g). Remove from heat and let cool slightly.
In a high-speed blender, combine the remaining 670 g fresh watermelon, the cooled reduction, corn syrup, sugar, lime juice, lime zest, and salt. Blend on high for 30–60 seconds until completely smooth.
1000 g seedless watermelon, 75 g light corn syrup, 25 g granulated sugar, 2 tbsp fresh lime juice, 1/4 tsp kosher salt, Zest of 1 lime
Pour the mixture through a fine mesh sieve into a bowl or pitcher. Use a spatula to press it through, discarding any pulp or fibrous bits. This step ensures a silky texture.
Cover and refrigerate for 2–3 hours, or until very cold. (Overnight chilling works too.)
dd the chilled base to your ice cream maker. Churn according to manufacturer instructions — usually 20–25 minutes, until thick and slushy like a frozen margarita.
Transfer to a lidded container. Smooth the top and press a piece of parchment directly onto the surface to prevent ice crystals. Freeze for 2–4 hours, or until firm and scoopable.