Using a screwdriver (or awl) and a hammer, poke through the eyes of the coconuts, and drain out the liquid through a strainer into a mixing bowl. There should be a total of about 2 cups. Set the shells aside.
Add the evaporated milk, gin, sugar if using, cinnamon and nutmeg to the coconut water. Stir to dissolve the sugar.
Refrigerate the mixture until cold, at least 2 hours, or cool mixture down with ice.
To serve, carefully pour the mixture through a funnel into the reserved coconut shells. Insert straws.