In a blender, blend the lime zest, fresh lime juice, sugar, and 3 cups of cold water.
Add the sweetened condensed milk and blend again.
Add remaining water and stir well. Serve well chilled over ice.
Although I used a blender, you could skip the blender and mix all the ingredients together in a pitcher. If you choose to make it in a pitcher, it will take a bit of extra mixing to make sure the sweetened condensed milk is thoroughly combined.