"I am not worried, Harry, I am with you."
-- Dumbledore (HBP26)
Harry and Dumbledore enter the cave where Voldemort has hidden a Horcrux. After swimming to the entrance, Dumbledore spatters it with his blood to gain entry and they take a hidden boat across a lake. Protecting the Horcrux is a potion that Dumbledore drinks, though Harry must force him to; after they obtain the locket Inferi erupt out of the water because Dumbledore needs water and the only way they can get it is by disturbing the lake. A weakened Dumbledore casts a fire spell to repel them as they escape.
Commentary
Notes
The potion protecting the locket gives off a green glow that can be seen in the darkness from the water's edge. This is reminiscent of the iconic green light in The Great Gatsby by F. Scott Fitzgerald. Like Jay Gatsby, Lord Voldemort rejected his humble origins, reinvented himself with a new name, got ahead by dishonest means, and obsessively pursued an obsession that eventually lead him to his death. -BB
Harry displays a great deal of selflessness during this adventure, offering to bear the hardship himself each time the situation requires Dumbledore to be harmed.
Harry and Dumbledore taking the tiny boat to the little Horcrux island is similar to Hagrid taking a small boat to the Hut-on-the-Rock (PS4). -BB
The event also bears similarities to Harry retrieving the Sword of Gryffindor from the dangerous waters of the lake in the Forest of Dean (DH19). -BB
As soon as Harry suggests using the Summoning charm to retrieve the locket he realizes it was a silly idea. But Dumbledore suggests Harry try it. Harry says, "Accio Horcrux!", and the only thing that happens is that one of the inferi reacts to the magic by jumping out of the water. However, this does not necessarily confirm Harry's belief that the Summoning charm would not work on the locket he and Dumbledore retrieve since this locket turns out to be a decoy and not a real Horcrux. -BB
Of the inferi, Dumbledore says, "like many creatures that dwell in cold and darkness, they fear light and warmth..." This alludes to Voldemort not knowing love or friendship, and being defeated by it. Dumbledore clarifies that fire will repel the water-dwelling inferi. This is interesting because fire is the element loosely aligned with Gryffindor, while water is loosely aligned with Slytherin. -BB
It is interesting that the potion basin resembles Dumbledore's pensieve, because the potion appears to force Dumbledore to revisit an experience he may have wished to forget. -BB
After hearing from Aberforth about the duel that killed Ariana, Harry is sure that it was that duel with Grindelwald and his brother that Dumbledore was reliving while drinking the potion (DH28).
Throughout DH, it is revealed that Dumbledore has controlled and manipulated people (especially Harry) to achieve certain ends. The roles reverse inside the cave when Harry must lie to and coerce Dumbledore, forcing him to endure enormous suffering so that what Dumbledore might call "the greater good" can be achieved. -BB
Perhaps one of the inferi Harry and Dumbledore encounter was Regulus Black, who himself fell victims to the creatures in the cave's lake (DH10). -BB
Dumbledore is murdered not long after the potion in the cave drives him to plead to be killed (HBP27, HBP28). -BB