The reponse to this petty requirement should have been this: on every single flight order bottles "to sign for" from ops. A couple of captains I've flown with do this now. They ask for and sign for a bottle of water for each crew member for each leg they fly, every single day they fly. There's water everywhere.
If the company's position is "it is no problem to get water, we just want you to sign for it" then fine, turn it around and order bottle for each crew x 1000 legs per day. No problem, right? Thank you very much.
This should have been the union's formal response.* The company would have promptly rescinded the letter completely and learned a valuable lesson going into negotiations: if you're going to play games with the pilot group, then expect games back from the pilot group. And these games will cost you money.
*This would not have violated RLA "established" behavior regarding the contract. It could have been done.