Cut the watermelon in half horizontally so there are two halves. These will be our "cups".
Cut the inside of the watermelon flesh horizontally then vertically. The bigger the chunks, the easier it will be to grate later. Be careful not to pierce the skin. Scoop out the watermelon chunks and freeze them for at least five hours but I like to do overnight. The watermelon needs to be rock solid.
Wrap the watermelon halves in plastic wrap or store in an airtight container. Place in the fridge until ready to use.
Once the watermelon chunks are frozen, grate the watermelon into the watermelon boats using the large side of a cheese grater.
Top with a squeeze of lime and a drizzle of condensed milk. Eat and enjoy immediately!
Notes
If you don't want to use the watermelon shells as cups thats fine! You can always just cut up the watermelon and shave it into a cup or bowl.