Prepare the graham cracker crumble as directed in the recipe card. Set aside.
In a blender, combine the condensed milk, lime zest, and salt. Blend briefly to release the lime oils.
Add the lime juice and water. Blend until smooth.
Start with 3 cups of ice and blend on high until thick and slushy, adding the remaining cup if needed. Taste and adjust the sweetness or tartness as needed.
Serve immediately in chilled glasses.