Place a 9-by-5-inch loaf pan in the freezer.

Puree watermelon in a food processor or blender until smooth (you should have about 2½ cups).

Combine 2¼ cups of the watermelon puree, lemon juice and salt in a small saucepan. Bring to a simmer over medium heat. Cook until reduced by half, about 15 minutes. Transfer to a shallow bowl and let cool completely, about 15 minutes.

Meanwhile, whip cream until it forms stiff peaks. Drizzle in the remaining ¼ cup watermelon puree and continue to whip until incorporated, about 30 seconds. Fold sweetened condensed milk into the whipped cream until incorporated.

Pour half the whipped cream mixture into the loaf pan, then drizzle half the watermelon reduction in a figure-eight pattern over the top. Top with the remaining whipped cream mixture and drizzle the remaining watermelon reduction in figure-eight pattern over the top.

Cover with plastic wrap and gently press down so it lies on the surface. Freeze until solid, at least 6 hours and up to 1 week. Let stand at room temperature for 5 minutes before slicing. Garnish with more chopped watermelon and mint, if desired.