Pour the coconut water into a large measuring cup.
Zest the limes and add the zest to the measuring cup. Cut 1 lime in half and squeeze the juice into the measuring cup. Finally, add the simple syrup to the measuring cup and with a small whisk, whisk everything together.
Pour the mixture into a glass 8x8-inch pan. Carefully place it uncovered into the freezer for at least 6-7 hours. Remove the pan every 2 hours and scrape with a fork. It's okay if some of it is still liquid..just scrape and place back in the fridge and repeat. You'll eventually end up with a consistency like the above photos!
Scoop out into bowls and serve immediately. Cover the remainder with foil and place back in the freezer for future eating.
Notes
To make simple syrup, take equal parts water and equal parts granulated sugar and bring to a low simmer. Whisk until sugar granules have dissolved. You have simple syrup! :)