Ah, here was a good spot: a little hollow in the ground, with a pair of trees nearby that would hide him from view while giving him a gap to watch through.
It took only a few moments with a wand to dig the soft ground into a pit, where he carefully placed the sword of Gryffindor. Then he conjured some water and filled the pit, so that the sword lay glittering at the bottom of a pool. The surface quickly began to ice over in the cold air. Snape added a Freezing charm, and soon the pool was covered with thick grayish ice, but the sword remained clearly visible. Good. It would not be too easy to get, but even a boy of Harry Potter's mediocre skills could retrieve it.
Snape whispered, "Expecto patronum," and the beautiful, painful silver doe appeared in front of him. He had gotten used to it by now, but it still hurt him every time he looked at her. This time, though, it was perfect ... Potter would be sure to follow her, how could he resist? It was his own mother's image, in a way.
Potter, like Dumbledore, was a sentimentalist. He believed in the power of love. He trusted his emotions far too much, but for once that suited Snape just fine. The goal was to get the boy here so he would find the sword.
He pointed his wand back toward Potter's camp, willing it: Find Potter, and bring him here. The silver doe darted away, bounding gracefully through the trees. Snape had to keep half his mind on his Patronus, but in the meanwhile, he melted the snow around the pair of trees where he stood and concealed himself, watching the pool.
Sure enough, in about fifteen minutes, along came Potter, following the doe. Snape smiled to himself just a little; in the silvery glow of the Patronus, the boy's face was clearly entranced. And fortunately, he was alone. The girl, Hermione, was probably asleep.
Snape allowed his Patronus to vanish, and Potter was left in the dark. He lit his wand and gazed down into the pool. Yes, he'd seen the sword. Now he seemed to be thinking about how to get it. Finally he stripped off most of his clothes, down to his underwear, but didn't remove a large gold locket from around his neck. Snape frowned; was that a green letter S glittering on the locket? What did it mean? Why was the boy wearing that thing, and why was he so reluctant to let it go?
Potter broke the ice with his wand and dived into the pool. Through the splashing water it was not easy to see, but he seemed to be having some trouble. He did not resurface.
Snape counted seconds to himself, waiting. Couldn't the boy do anything right? A smarter wizard would at least have used a spell to warm the water first. But then, perhaps it was just as well this way; diving into freezing water required more bravery...
Twenty seconds passed. Thirty. Forty. Snape hesitated. If he came out of his hiding place and rescued the boy himself, it'd ruin the plan, but it was better than letting Potter die too soon ...
Then, wonder of wonders, Ron Weasley came running out of the trees. Harry's best friend, of course: he must have been there for several minutes, but Snape, intent on Harry, hadn't noticed him. What was he doing here?
Ron had already dropped his rucksack. He dived into the pool after Harry with all his clothes on.
Snape breathed a sigh of relief. Now everything would be all right. Ron quickly heaved the unconscious Harry out of the pool, then dived for the sword himself. Perhaps Dumbledore had had a good reason for giving Weasley his treasured Deluminator after all...
As Snape Disapparated, he could only wonder just how Dumbledore had known that Ron would show up at exactly the right moment.