This one really annoys me because I've even had some nurses telling me that I should be drinking 2 litres of water a day and I have to explain to them that unless you have damage to your hypothalamus or some other problem with homeostatic feedback mechanisms (e.g. drugs, renal problems) - your thirst will tell you how much you need to drink.
Also as you point out drinking too much water can be dangerous (although in a healthy individual 2 litres probably won't cause a problem, in certain diseases particularly where your have poor renal function the threshold may be much lower).
I think in part this myth may have been perpetuated by bottled water companies.
Your body will indeed always let you know how much you need. The same holds for many of the every-day-needs. But myths are hard to fight. Educating people is the only way out (as you did with these nurses)!
Exactly!
That is unfortunate that some many health professionals such as nurses and doctors still perpetuate this nonsense :-)
You are probably one of few doctors who debunks this :-)
I also think that water corporations have largely contributed to this myth. Just like tobacco companies 70-80 years ago used to perpetuate the bullshit that cigarettes are great medication :-)