His foot must have slipped.
The beer in his hand slips free and the glass crashes to the aqua tile, glinting off the chlorinated surface as it shatters into a thousand pieces.
Or something pulled him – backwards.
It's like slow-motion as the smile on Dean's face transforms from the smart-ass comment he was about to make into something like horror as he feels himself falling, and his arms fly out to catch at nothing.
When his head hit the concrete edge of the pool, it was the sound, more than the sight of blood spanning out like ribbons through the clear water, that made Sam's stomach clench and his heart leap into his throat.
He lurched toward his brother floating face-down in the shallow end of the pool. He jumped in, feet first, frigid water hitting hard and stealing the breath from his lungs as soon as his shoes connected with the slippery concrete below, and plastering his clothes to his skin, weighing him down like too many extra limbs. He fought the water's extra pull along with Dean's still form, and wrestled his brother's unconscious body up over the ledge of the pool, trailing rivulets of pool water from his saturated flannel shirt, jeans, and boots.
He thought he could hear himself ordering someone to call an ambulance, not sure where his own voice was coming from.
He heard someone ask if the man was breathing, and Sam almost lost it. Because he wasn't. And couldn't lose Dean, not like this. Not in a stupid hotel pool drowning.
And then all at once, his father might as well have grabbed him by the shoulders and smacked him hard across the face, because he snapped back together, pressing his fingers against the side of Dean's throat and digging in to try and find a pulse. Dean's skin was too cold, too pale, and there was no rush of blood where there should have been a pulse beating under his fingertips. Only the slow trickle of blood from the gash in his forehead, bright red smearing and spreading as it joined with the wetness of the pool water clinging to Dean's scalp.
"No. No, come on!" he urged Dean.
He yanked Dean's shirt apart, baring his chest, threaded his fingers together and started counting out compressions. There were tears in his eyes. This wasn't right. When he saw his brother's chest, it was the familiar sight above a towel every morning after Dean stepped out of the shower. It was the thing he smacked when Dean teased him. The thing he shoved against when they fought. The solid wall of comfort he'd laid against when he'd had a fever and couldn't get out of bed to throw up. He wanted to throw up now. He wanted this not to be happening.
He stopped and tilted Dean's head back, pinched his nose shut and pressed his mouth over his brother's pale, bluish-tinged lips, forcing air into Dean's lungs. He breathed in again through his nose, and then again for Dean, silently begging him to catch on and pick up the rhythm on his own.
He felt a shudder run through Dean, and then he turned to his side and coughed. Sam gasped with relief and dropped back on his heels. Applause erupted from the small group of onlookers that had gathered outside the hotel, and Sam looked up in shock, not even aware that they had drawn a crowd.
A fist connected with his jaw, and Sam tumbled backwards.
Dean, still too pale, dropped back against the tiled surface along the edge of the pool, breathing hard, eyeing Sam and massaging his right hand.
Sam touched the sore spot where he'd been punched, tentatively working his jaw. "Dean? What the hell!"
"Rule number one, Sammy." His voice was rough, and Sam could see the interplay of fear and gratitude in his eyes that he'd never voice. "Mouth-to-mouth requires a hot lifeguard."
Sam shook his head, pushing himself back up to his knees beside his brother. "Man…seriously? You can save yourself next time!"
"I mean, doesn't have to be Baywatch hot. Just passable hot."
"You know your heart stopped, right?"
Dean held out his hands. "Rules are rules. I don't make the rules. And I mean, sorry man, but you just don't qualify."
"Oh, okay, Dean." Sam rolled his eyes. "Sorry I violated 'hot lifeguard rule number one' by saving your freaking life."
Dean nodded solemnly, then looked up at Sam and frowned, all traces of levity gone. "Hey, Sam. Pretty sure something grabbed my leg."