I am not suggesting water is not important. That being said, 64 oz is at best a guesstimate, since individual needs vary. Also, most foods have a water content that is considered to be a part of your daily intake.
As for the US government information, you literally just posted about the WHO and the US being at odds regarding sweeteners. Not to mention all the bad nutritional information we received over the years. (Are eggs good for you this week?!? LOL) We like to pretend the US has the highest standards but many European nations don't even allow diet soft drinks since they're linked to cancer. So ..who's to say who you can really trust?!?
Also, we love to say we cannot give medical advice .. So, I'm leaving it to the professionals!! Listen to YOUR surgeon!!. I promise he is more informed than your local bariatric patient. Everyone gets their info from somewhere .. I'll take mine from a team of medical professionals, rather than an internet search. Just my opinion.