after the latest iphone app update (Feb 2022), when you manually add the first "cup" to the hydration app, the number remains on 0, BUT, if you sync to the watch, the watch hydration will show 1 cup.
after this first cup, obviously, the phone and the watch remain out-of-sync - even the daily goal shows (for me) 14 cups on the phone, and 13 on the watch.
for the record, the previous app version (probably since sept/october 2021) had worked OK - meaning that the sync between the phone and the watch worked most of often than not...