I guess
this was the page? (I found the link on the FAA's site). To me, it seems you can only have water with you if it's a medical requirement. The container must fit inside a quart-sized ziplock bag.
Of course, it may vary from airline to airline, and from airport to airport.
If it helps: flying from the UK, you have to surrender drinks bottles at the security line. However, there's the chance to buy more drinks at the duty-free after the security line and on the plane. Flying back out of Amsterdam, the same rules apply, although the security check is right before you go onto the plane and duty-free drinks get security sealed so you can take them through the checks.
If you're getting conflicting advice from the airline, I guess you could just take the risk and bring the water with you. The worst that could happen is that they confiscate it from you and let you get on the plane anyway, right? (Although on second thoughts, maybe the worst that could happen is that the security guy gets out the KY and rubber gloves to check that you're not concealing additional beverages about your person)