Anonymous wrote:1.5 bottles a day? That's wasteful? That's not even as much as she needs to drink a day. If you're really that concerned with her excessive water drinking buy her a reusable water bottle and a Brita pitcher.
OP said that they drink filtered water at home already, maybe one of the built-in ones at the kitchen sink (those are the best and filter everything you can imagine, you really can't drink water any cleaner than from those). The nanny just doesn't use it.
I take a few old water bottles and fill them with that type of filtered water then put them in the fridge. I like my water really cold, or else I don't drink as much of it, and I like to have it be in a bottle with lid that I don't have to worry about spilling and can take with me at any time (and recycle if I don't want to carrying it around anymore when empty). Try offering that as a solution. Label a few bottles with her name so no one else touches them (which is the concern with having an already opened bottle, you don't want others to share them). If you have some empty ones that can be used to replace ones in circulation, then she can have a few to use daily, take out with and recycle, or replace once banged up. I would use some soda bottles or anything of a thicker plastic actually, for durability since water ones are usually really thin and don't last long when reusing.