Although his sick father begs him for water, Eliezer does not want to give it to him. His father is suffering from dysentery, and Eliezer knows enough about the camps to realize that the dysentery is caused by the dirty water.
Because there was no sanitation to speak of in the camps, the prisoners defecated into buckets or into holes dug in the ground. Because the prisoners very seldom had a chance to bathe, their bodies and clothes got dirty and picked up fecal matter from all over the camp. It was then very easy for bits of fecal matter to get mixed in with the water supply.
Water contaminated with feces leads to dysentery. Dysentery is characterized by diarrhea caused by the human body trying fight off an infection in the intestines, which is brought on by eating something toxic. The diarrhea from dysentery is debilitating and leads to dehydration. The dehydration leads to thirst, but if polluted water is the source of the dysentery, drinking it will only start a renewed cycle of dysentery and distress.
I knew that he must not drink. But he pleaded with me so long that I gave in. Water was the worst poison for him, but what else could I do for him? With or without water, it would be over soon anyway.
Eliezer continues to give his father water because he so terribly thirsty, and it is the only thing he can do for him. He knows it is too late and that his father is going to die. Eliezer realizes that his father has died when he wakes up one morning, and his father's bunk is empty.