In Half Blood Prince, Dumbledore drinks Voldemort's potion; Harry tries to fill up the cup with water, but it keeps emptying. He has to get water from the lake. Why couldn't Harry conjure water to pour straight from his wand into Dumbledore's mouth?
It seemed to be the act of bringing the water (that wasn't from the lake) close to Dumbledore's mouth that caused it disappear.
"But I had some -- wait -- Aguamenti!" said Harry again, pointing his wand at the goblet. Once more, for a second, clear water gleamed within it, but as he approached Dumbledore's mouth, the water vanished again.
Harry Potter and the Half-Blood Prince - Chapter 26
It is possible that using Aguamenti directly into Dumbledore's mouth would have worked. However, it is my opinion that Voldemort would have thought of this and it was more an area charm that prevented any water other than the lake's to enter the potion drinker's mouth. With that said I'm not even positive that the lake water could be drunk without disappearing either since before Harry is able to give the water to Dumbledore the army of Inferi were attacking.