system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] In Harry Potter and the half blood prince how did dumbledore know he had to drink all of it? Why not reach in to it and take the horcrux?

Also, what was that in the thing with the horcrux?

Edit: Also, what were those things in the water?
gpt
Dumbledore knew Voldemort very well; psychologically, he may have known more about Voldemort than any of his close circle.  So after checking the basics of the potion, proving that it couldn't be vanished, transfigured, or moved, he knew that Voldemort's intention must have been to harm the person who found his secret.

So naturally, the potion must be drunk fully, and naturally it would do something horrible to the drinker - Voldy would want him to suffer.

The things in the water were Inferi - basically zombies.  Once the potion was drunk, it created a great thirst.  The cup was bewitched not to hold any water except from the lake, and disturbing the lake released the Inferi.  It was a nasty trap that could only be solved by two people - something Voldemort couldn't understand.  By putting his trust in another, Dumbledore escaped in a way Voldemort would never have thought of.