In a small saucepan over high heat, bring 1 cup water, sugar, and lime zest to a boil; stir until sugar is dissolved.
Strain into a blender and add 3 cups cubed seeded watermelon. Whirl until smooth and pour into a 9- by 13-in. metal baking pan. Add 3 more cups watermelon to blender along with port and lime juice; whirl until smooth, add to pan, and mix well.
Cover and freeze until mixture is beginning to freeze at the edges, about 2 hours. Stir, cover, and freeze until solid, about 6 hours. Just before serving, scrape into soft clumps with a fork. Spoon into chilled glasses or bowls.