Whip the butter in the mixer until light and fluffy. (Stand mixer is easiest.)
Add in 6-7 cups of sugar. Whip until combined, stopping to use spatula to pull away dry ingredients from inside of bowl.
Pour in coconut water. Mix until blended.
Add in remaining 1-2 cups of powdered sugar and mix again until blended.
If buttercream is too dry, add in 2-4 more tbsp of coconut water. Mix until fluffy.
Add into piping bag with tip of choice.