if you “just want to hydrate yourself for health reasons”, that is לצמאו.
They’re not synonymous, though most often overlap.
You are assuming, for some reason, that it’s not possible for a person to drink water for hydration unless they are dehydrated. This is not true. Some people drink water when they’re not thirsty because they are told to drink a certain amount of water every day.
Another ramification would be before a taanis. Often, someone will drink water before a taanis to give extra hydration. Unless they’re thirsty, they would not recite a brocha.
Where does it say anything about flavorless food? The Mishnah and Gemara mention water.