Your best bet is to get what is known as a Living Will. This is a legal document that details your treatment wishes in the event that you are incapacitated and are unable to make your wishes known to family and health care providers at the time. It is a bit more complex than a Power of Attorney for Health Care (in which case you designate someone to authorize treatment on your behalf, and assumes that this person knows you well enough to do what you would want done).
You can cover things such as the extent of the treatment you wish to have (or not have as the case may be). You can get into things like whether you want to be intubated or be on a ventilator, whether you want CPR performed in the event of cardiac arrest. Whether you wish to have pain controlled or other comfort measures. Whether you want tube feedings or IV fluids. Whether you want your life to be maintained until certain friends or family members can be present to say their goodbyes.
In the hospital setting, I have never seen "DNR" to mean that water is withheld from a patient. What it means is that the person does not wish to have CPR, intubation or assisted ventilation in the event of respiratory and/or cardiac arrest.