Combine the watermelon, sugar, lemon juice, and salt in a large mixing bowl; stir to coat evenly. Cover the bowl with plastic wrap and refrigerator for 30 minutes.

Blend the mixture in a blender until smooth; return to the bowl.

Pour the cold water into a saucepan. Sprinkle the gelatin over the cold water; let stand 1 minutes. Place the saucepan over low heat; cook for 2 minutes. Stir the gelatin mixture into the blended watermelon mixture. Add the heavy cream; beat with an electric hand mixer at medium speed until the mixture is fluffy.

Transfer the mixture into an ice cream maker and freeze according to manufacturer's directions until it reaches "soft-serve" consistency. Transfer ice cream to a one- or two-quart lidded plastic container; cover surface with plastic wrap and seal. For best results, ice cream should ripen in the freezer for at least 2 hours or overnight.