Adrenaline? or swimming lessons?

When did Harry learn to swim?

In the fabulous cave scene at the end of The Half-Blood Prince, Harry swims through treacherous sea waters, from the rock onto which he and Dumbledore had just apparated, across a narrow stretch of water and into a fissure that eventually leads to a set of steps onto a rocky shore in the middle of a cave.

Rowling says that Harry was weighed down by his waterlogged clothes as he swam, that he "struck out" to follow Dumbledore (who, quite charmingly, was doing "a perfect breaststroke" with his wand held between his teeth), that he "continued to swim" in the "icy" water with "benumbed fingers" before seeing Dumbledore "rising out of the water ahead" and then following him.

What with the dangerous, icy water, the weight of his clothes, and the fact that Dumbledore is able to get quite far ahead of him, this sounds like quite a challenging swim.

Even more impressive, on the way back, Harry must carry the weight of a weak, faint Head Master as he swims back from the cave to the rock before apparating back to Hogsmeade.

It's an excellent, exciting part of the novel and leads directly into the incredible climax involving Death Eaters in Hogwarts.

But when did Harry learn to swim?

In book four, The Goblet of Fire, Rowling writes the following passage to describe Harry's reaction upon deducing that the second task in the Tri-Wizard Tournament would involve swimming into the lake on the grounds of Hogwarts:

But [Harry] suddenly realized what he was saying, and he felt the excitement drain out of him as though someone had just pulled a plug in his stomach. He wasn't a very good swimmer; he'd never had much practice. Dudley had had lessons in their youth, but Aunt Petunia and Uncle Vernon, no doubt hoping that Harry would drown one day, hadn't bothered to give him any. A couple of lengths of this bath was all very well, but that lake was very large, and very deep...

In two years, Harry has gone from someone whose stomach drops at the thought of being required to swim any distance to someone who can swim a fair distance through frigid, dangerous waters, carrying the weight of a fully-grown adult who is incapable of swimming on his own.

I guess you could argue that, when the need is that great, adrenaline would permit Harry to swim that distance, carrying Dumbledore... but it seems our beloved author missed something between these two stories, doesn't it?