Ok, I sincerely apologise. I just came from another thread talking about how people like the rudeness here and I think it made me misconstrue your intent, so please accept my apology.
Are you asking specific to water intake? Because Garmin will cite different associatations depending on the metric, e.g. heart readings and zones are measured and intepreted via American Heart Association as confirmed here:
https://www.garmin.com/en-US/garmin-technology/health-science/
Hydration is measured, according to Garmin Connect (the app you use for your watch) as follows:
"Our default daily hydration goal is derived from the recommendations of several credible health organizations, including the US National Academty of Sciences. The goal is intended to guide your water intake through liquid consumption only, not food (on average 20% of your water intake comes from food).
You can choose this default goal or create your own daily goal. If you choose the auto goal option, your hydration goal will increase dynamically based on estimated sweat loss for timed activites you perform. Sweat loss is estimated by analyzing exertion and ambient termperature.
Your body needs water to survive, and it makes up to 60% of your body weight. In general, thirst is a good indicator of when to hydrate. You can also use urine color as a guide.
It goes on a bit more on this point then under, "What to Hydrate with,"
While water is often regarded as the best choice for hydration, the reality is that almost any non-alcoholic liquid is acceptable. Caffeinated beverages produce a moderate diuretic effect, but their net impact is hydrating rather than dehydrating.
Hope that helps and apologies, once more. I touch typed that info for you, using my phone and Garmin app, so please excuse typos.
I will try to not default to a defensive position when reading and responding, in future! Thank you for being so diplomatic regarding my error.