Please note: This begins right in between scenes in the episode "The Gates of Avalon," so it's probably not going to make any sense unless you've seen that episode.
Merlin heaved the unconscious Arthur out of the lake, wondering, Now what? He really hadn't thought beyond the point of save Arthur from evil Sidhe people. Also, the full armor Arthur was wearing made him incredibly heavy, even though he was still half-submerged in the buoying lake.
And that was the least of his problems. Looking down at him, Merlin realized in panic that the prince wasn't breathing. "Arthur?" he called frantically, towing him as fast as he could back to shore. "Arthur, can you hear me? You have to wake up. Please." There was no answer.
As quickly as he could, Merlin towed his friend through the water and dragged him up onto the dirt, sand, and rocks by the side of the lake. Turning him onto his stomach, Merlin hit him as hard as he could on his back with both fists, trying to be rhythmic. "Come on..." he muttered desperately, over and over. "Come on, Arthur..."
Water came pouring out of the prince's mouth onto the ground. After several moments when the flow lessened, he coughed convulsively and started to breathe again, gasping and coughing although his eyes were still shut.
Relieved, Merlin turned his attention to removing Arthur's armor. He glared at his cold, numb fingers in annoyance as they fumbled, trying to undo the wet fastenings. Even he wasn't normally this slow. If Arthur had been conscious, he would have been haranguing him mercilessly by now. Some time today would be nice, Merlin, if you don't mind...
Finally, he got all the armor off and onto a loose pile on the ground beside Arthur. Part of him wondered, worried, whether his friend should have woken up by now, but he had been under an enchantment that had been violently removed, not to mention him having been nearly drowned; besides, at this particular moment Merlin was more worried with removing the chain-mail coat Arthur wore under his armor. The metal so close to his skin would only make him colder and help retain the moisture of the wet clothing underneath. The day was cool, and it was late afternoon, edging quickly into early evening; had Merlin been paying attention, he would have noticed that he himself was shaking violently in his drenched clothes.
Awkwardly, he heaved Arthur into a sitting position and maneuvered his lifeless arms in a way that allowed him to pull the ungainly metal garment over his head. As an afterthought, he pulled off Arthur's drenched shirt as well, and laid him down gently on the ground, putting a friendly hand on his shoulder as though trying to reassure the unconscious man. He got to his feet, stumbling slightly, and searched the nearby lakeside for the dry jacket that he knew he'd left there. Where had he put it? It had only been a couple of minutes ago, but he couldn't remember.
But he knew it had to be nearby, so he just searched the nearby lakeshore until he found the familiar brown object. Returning to his friend, he carefully draped it over his chest, tucking it around him. Unfortunately, Merlin was far too skinny for the jacket to fit on Arthur in the proper manner, but it served well enough at a blanket nevertheless.
Merlin sat down beside Arthur and stared at him for several minutes before realizing he had no idea what to do next. Again, he hadn't seemed to have thought beyond a certain point. He was angrily at himself - normally, he was nowhere near this shortsighted, no matter what Arthur might frequently say to the contrary. Arthur seemed to be relatively out of danger of drowning, hypothermia, evil magical creatures or any other similar immediate danger as far as Merlin could see, but he should still have Gaius check him as soon as possible.
The only question was, how to get him back to the castle? Merlin realized with chagrin that he had no reliable way of getting the prince back to Camelot by himself. He considered trying to carry the heavier prince back all the way he had run in the swiftly-falling darkness, but that was ridiculous. And he could hardly leave him helpless here without protection while he went back to fetch horses.
Disgusted with himself, Merlin headed off into the trees by the lake and collected branches, both dry and green ones, then dug a small well in the dirt a few feet from Arthur's still form and used his magic to light a fire. The resulting small blaze sent smoke spiraling into the air high above them. Hoping someone friendly would see his signal, Merlin turned to Arthur to make sure he was reasonably comfortable, then sat down on the other side of the fire. Suddenly, he was completely exhausted. Heedless of the chill night breeze, his still-wet clothes, and the complete lack of any kind of covering for himself, the warlock huddled into a small ball by the fire and was asleep in seconds.