How to Make It
Stir together watermelon juice, sugar, and kosher salt in a small saucepan. Cook over medium, whisking often, until sugar has fully dissolved, 3 to 4 minutes. (Do not simmer.) Remove from heat, and whisk in coconut milk, sweetened condensed milk, heavy cream, and lime juice until fully incorporated.
Pour mixture into freezer container of a 1 1/2-quart ice cream maker, and proceed according to manufacturer’s instructions until ice cream has the texture of soft serve, about 1 hour. (Instructions and times may vary.) Stir in lime zest.
Quickly transfer ice cream to a freezer-safe container; press parchment paper directly onto surface. Cover container, and freeze until firm, at least 6 hours. Ice cream can be stored in freezer up to 3 months. To serve, garnish with fresh watermelon, flaky sea salt, and lime zest.