Cut the watermelon in half and first make a few watermelon balls with a melon baller (about 10). Put in an airtight container and place in the freezer. Peel the leftover watermelon, chop it in small pieces.
Transfer watermelon to a blender, add lime juice and 120ml (1/2 cup) coconut water. Blend until completely pureed and smooth. Taste and add as much agave syrup as you wish, depending on the sweetness of the watermelon.
Strain into a pitcher, using a fine mesh sieve. Add remaining coconut water and stir well.
Serve over ice, add a few mint leaves and lime slices, top off with 1-2 frozen watermelon balls.
You can make this ahead. Store it in an airtight container in the fridge and drink within a day.