I agree with Ward's conclusion that the potion is not meant to kill the drinker especially given the evidence of the other 2 victims of the potion. However, I think Dumbledore is wrong on his assumption of WHY the potion will not kill him.
Lets investigate, first we have Dumbledore's comment.
"Oh, I doubt that it would work like that," said Dumbledore easily. "Lord Voldemort would not want to kill the person who reached this island."
Showing that he doesn't believe the potion is meant to kill the drinker, here I think most of us agree that death from the potion is not the intention.
This is where I think Dumbledore's conclusions are slightly off.
"I'm sorry, Harry; I should have said, he would not want to immediately kill the person who reached this island," ... "He would want to keep them alive long enough to find out how they managed to penetrate so far through his defenses ..."
While The potion does seem to incapacitate the drinker, it also has a side effect, making the drinker incredibly thirsty. The cup/potion itself does not appear to allow any liquid into the victims mouth, unless it's from the lake.
What do we know about this lake? Its filled with inferi.
What do these inferi do when you drink from the late? Immediately attack and kill you.
“Kreacher needed water, he crawled to the island’s edge and he
drank from the black lake ... and hands, dead hands, came out of
the water and dragged Kreacher under the surface...”
We see Kreacher was attacked an dragged under the water by the inferi.
And he drank — all the potion — and Kreacher swapped the lockets —
and watched ... as Master Regulus ... was dragged beneath the
water ... and ...”
Regulus, while not specifically stating he drank from the lake, he most likely did because of his thirst, and was dragged into the lake by the inferi.
instinctively, the only way left to get water, because Voldemort had
planned it so...
He flung himself over to the edge of the rock and plunged the goblet
into the lake, bringing it up full to the brim of icy water that did
not vanish.
“Sir — here!” Harry yelled, and lunging forward, he tipped the
water clumsily over Dumbledore’s face.
It was the best he could do, for the icy feeling on his arm not
holding the cup was not the lingering chill of the water. A slimy
white hand had gripped his wrist, and the creature to whom it belonged was pulling him, slowly, backward across the rock.
As Harry states, he knew instinctively what Voldemort had done, made it so you had to drink water from the lake, the water that activated the inferi, to attack and kill you. Which shows the intention of death from inferi, and not the potion, which would not keep the person who found the locket alive long enough for Voldemort to question them, as Dumbledore suspected.
