I just noticed this morning when I was entering my weight in GCM that next to the "+ Weight" button was a "+ Hydration" button.
Tapping the "Hydration" button allows me add a "cup" at a time, or quick add 1, 2 or 3 cups.
There's a "Settings" option within that page that allows you to customize Units, Daily Goal and the three "quick entry" amounts.
There's indication it will "sync with your devices" - not sure which devices are supported (I have a FR645) or if there is a watch app but don't see any data transfer just yet to my device.