For a healthy, robust person to suddenly be deprived of water can be uncomfortable. The sensation of thirst is severe, and fluid deprivation will result in very dry skin, mouth, lips, other tissues, etc.
A person severely debilitated by serious or terminal illness, on the other hand, will likely lose the desire to drink and eat as the body signals that it is dying. For this person, the sensation of thirst is likely to be mild. Dry mouths can be swabbed with glycerin or rinsed with special mouthwashes. Cracked lips and dry skin can be avoided or soothed with moisturizing agents.
In either case, after a few days, the sensation of thirst is actually replaced by a feeling of euphoria due to chemical changes in the body. The person might hallucinate or behave strangely, but the overall affect is usually calm. Pain is not typical in this phase at all. Eventually, the person will lapse into unconsciousness and die -- usually within two weeks.
For that person, to force fluids by intravenous, gastrointestinal, or nasogastric tube actually creates worse suffering and symptoms than it prevents. As the body approaches death, the kidneys are no longer able to process and expel the fluid. Fluid then backs up into the tissues, causing very uncomfortable swelling. Aspiration pneumonia is common and bedsores, actually, more likely to occur. If the tube for the delivery of the artificial nutrition and fluids is surgically implanted in the person's abdomen, infection and pain are likely. The only "benefit" to the tube feeding for this person is to greatly prolong their dying.
