Dumbledore and Harry Apparate to a cave the middle of a vast sea. Guess this is where they're going to find Horcruxes. What, you expected something less spooky?
Dumbledore sacrifices some blood to get access to the cave (eek), and then he and Harry set to work trying to figure out where this Horcrux thing is likely to be.
They spot it, but it's on a tiny island across the water that fills the cave. Because of course.
But look: Dumbledore conjures a boat out of the water and pulls it toward them. Man, he is good.
They take the boat to the island with the Horcrux and approach the, er, treasure (if that's what you want to call it).
The Horcrux is in the bottom of a bowl filled with some kind of potion, and Dumbledore realizes that he has to drink all of the liquid to get to it. He warns Harry that his job is to make sure Dumbledore sticks it out, even if the potion makes him beg for relief. This is stressful already.
Sure enough, after starting to drink, Dumbledore has trouble continuing, so Harry forces him to go on. It looks really rough. We're a little unsure if Dumbledore (and Harry, for that matter) will be able to take this.
Whew, that was painful, but they do it—all the liquid is gone, and Harry has retrieved the Horcrux, which is a locket.
Dumbledore is totally spent and begging for water. Harry tries to oblige, but he can't conjure any from thin air. So, he goes to the water they just crossed and considers getting it from there. Why does that seem like a really bad idea? Oh, jeez, now we're really stressed.
Harry seems a little worried about touching the water, too—and sure enough, when he tries to get a cup of water, a super creepy hand bursts out of the water and grabs his wrist. Using his wand like a flashlight, he sees that some skeletal creatures that have been living in this cave water are waking up and–oh, wow, there are a lot of them coming straight for Horcrux Island.
Despite his attempts to fight the creatures off, Harry gets pulled under water and held under by one of the evil skeleton monsters. It does not look good. But all of a sudden the creature lets go and Harry sees some fiery orbs shooting through the water. When he comes to the surface he sees that Dumbledore has recovered (phew) and is working his magic to repel the dangerous sea people.