Step 1: Puree the watermelon cubes in a blender until completely smooth. Strain through a fine-mesh sieve into a small saucepan to remove any remaining pulp or small seeds.
Step 2: Simmer the watermelon juice over medium heat for about 10-12 minutes until it has reduced by about half. This concentrates the flavor and removes excess water. Let it cool completely in the refrigerator.
Step 3: In a medium bowl, whisk together the cooled watermelon concentrate, sweetened condensed milk, vanilla extract, and salt until well combined.
Step 4: In a separate large bowl, beat the chilled heavy whipping cream with a hand mixer or stand mixer until stiff peaks form. Be careful not to overbeat into butter.
Step 5: Gently fold about one-third of the whipped cream into the watermelon mixture to lighten it. Then, carefully fold in the remaining whipped cream until no white streaks remain. If using chocolate chips, fold them in now.
Step 6: Pour the No-Churn Watermelon Ice Cream mixture into an airtight container. Cover and freeze for at least 6 hours, or ideally overnight, until firm.