Cut the watermelon in half and scoop out the fruit, reserving the watermelon juice. If desired, save half of the rind for serving.
Place the fruit on a baking sheet lined with parchment paper or a silicone mat. Put in the freezer for at least 4 hours. Keep the reserved watermelon juice in a container in the fridge until needed.
Once the watermelon is frozen, remove from the freezer and put in a blender (not all of it will fit, you will have to do it in batches).
To the blender, add juice from half of the lime, the salt, and about half of the sweetened condensed milk. Pour in about half of the reserved watermelon juice. Blend, pulsing if needed, until smooth.
Continue adding more frozen watermelon to the blender along with the additional sweetened condensed milk, lime juice, and watermelon juice to your preferred taste and texture.
Pour into the reserved watermelon rind or individual serving dishes and enjoy! Store any leftovers in a sealed container in the freezer.