Why
would Elie say "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 states that water is the worst poison for a person in his father's condition, suggesting that the water is filthy and would further aggravate the situation. He prefers giving his father some soup because it is boiled and offers a healthier option compared to the water they had at the camps.

they are direct opposites. they can both be corrupted. they are inseparable.

i would need the available options to be listed to give you an answer.