Dean hadn't really fallen asleep, per se. Between the memory of what had just happened, including watching Seamus's mum get killed, and the thought that Seamus had no idea where he even was, or what had happened...It didn't take long after he was alone and no longer petrified for Dean to curl in on himself and be overcome with worry. He tried to think clearly and got as far as no wand, no idea where he was, and Seamus's mother was dead.
A choked sound came out of his throat then. He had to find a way out of here, one way or another, but he was too drained to think how. Finally, after what felt like hours, his body mercifully shut down for him.
Then he was rudely awakened but a frigid splash of water that was so cold he couldn't help the yelp of pained surprise that escaped. He jerked up instantly, but his mind came awake slower. Everything crashed over him again when he looked up and saw that face. His nostrils flared and he set his jaw. This was unlikely to go well, but he was not going to die here. Seamus had lost too much already. Dean was going to make it out. He had to.