Different take on episode 4x16, "Linchpin", when Beckett's car takes a nosedive into the Hudson.

Dry Land

"This door won't open." Castle says with a tremor in his voice, continuing to rattle the handle in a futile attempt to escape the rapidly filling car.

"Yeah, mine won't either. I think the crash bent the frame."

Castle pounds against the glass with his elbows, trying in vain to break the glass. Each time his elbow connects with it pain flares up his arm, but he continues nonetheless, determined if nothing, to get his partner out of the car.

"We need something to break the glass." Castle puffs, giving his battered elbow a rest.

"My gun." Beckett says with a sudden realization. "Castle, I can't…I can't find it. I-I think it got knocked out when we got hit. Help me look for it."

Castle removes his seatbelt. Beckett tries to unbuckle hers, but it won't budge.

"C-Castle." She says, with fear in her voice.

He stops running his hands through the water to look up at her, noticing the fear in her bright green eyes.

"I'm stuck."

"What do you mean, you're stuck?"

"I mean, my belt is jammed and the seat won't move."

Both of them put their hands on the buckle in an ill faded attempt to free her.

"Okay, what about a knife? Do you have a knife?" Castle says, looking from her to the buckle.

"Yeah," Beckett says with a glimmer of hope that disappears as quickly as it appeared. "In the trunk."

They both look down for a moment at the water seeping into the car beneath them as the car continues to sink.

"Okay, look, we're running out of time. I need you to see if you can find that gun, and we'll just shoot the belt off."

Castle reaches his hand under the chair, rummaging around for the weapon.

He gives off a frustrated grunt, "I can't feel it."

"Castle…" Kate says, the fear clear in her voice.

"Okay, hang on. Hang on." Castle says, looking at her with both fear and determination in his eyes.

Castle grabs a flashlight from under the seat, and looks over at her with resolve in his eyes.

"Just stay here." He says.

She gives him an incredulous look and gestures to the belt she is currently stuck under. He is paying no attention, too focused on the task at hand to realize his useless comment.

Castle inhales deeply and dives under the water now filling the back seat. He searches under Beckett's seat. Seeing the gun he tries to grab it, but it's stuck between some of the seat wires.

Meanwhile, Beckett is sitting in her seat as the water rapidly rises up her chest. She grabs the wheel as the panic sets in. Unable to get the gun free in time, Castle rises back to the surface in the car, inhaling a deep breath into his burning lungs.

"I found it." He says, taking another deep breath and going back for the gun.

Beckett desperately tries to remove her belt again. Castle struggles to pull the gun free as Beckett continues to fight with her seatbelt. Exhausted, she stops to take a breath, leaning her head back against the seat. She listens for the sounds of the water lapping as Castle moves around, but she hears nothing. Looking in the rearview mirror, the only sight she's met with is the back of her cruiser, nearly filled with water.

"Castle?" Beckett says quietly into the silent air of the cruiser.

She's met with no reply, and her panic instantly increases. She puts her hand out toward Castle's seat under the water and feels nothing. Terrified, she calls out for him again, only to receive no response.

Beckett sits alone for a moment as the water continues to rise. She tries to use her elbow to break the window, but it doesn't work. She struggles with her belt, but again, it's an ill-faded attempt. Kate lifts her chin up to gasp for air above the waterline as it slowly rises past her neck. With nothing left to do, she grabs the wheel and holds her breath, waiting for the inevitable.

'This is it.' She thinks to herself. 'I'm going to die alone in this car.' Alone. With that one word her mind drifts back to the man who is currently somewhere in the car, probably unconscious, or worse. She closes her eyes, a tear escaping, only to be masked by the water that has completely submerged her.

Her hands relax on the wheel and float in the car. As she drifts into unconsciousness, she thinks she feels a movement in the water near here. As the darkness pulls her in, something ghost across her fingers, but it's too strong, pulling her in until everything fades.

Gunshots fire in the car as it sinks, the last few shots blowing out the back window. Water rushed in through the now shattered glass, filling the car completely. Castle wrapped his arms around her, hauling the dead weight of her unconscious body towards him. After getting a secure grip on her, he used his feet to push off of the seat and swim towards the surface. Kate's body weighted them down as Castle struggled to pull them towards the surface where they'd finally be able to take in a much needed breath. Castle kicked his legs furiously as they slowly ascended. After what seemed like forever, but in reality had only been a few seconds, the pair broke the surface of the murky water. Castle gulped large breaths of air, rapidly sucking in the oxygen to fill his burning lungs. He struggled to keep Kate's head above the surface as the combination of exhaustion and the cold water set in.
He pulled her to him tighter, leaning close to whisper in her ear,
"C'mon Kate, stay with me. I'm gonna get you out of here, just hold on."
Castle swam towards the pier, looking for a way to get them out of the water. About ten yards away he spotted a ladder attached to the pier. He kicked his legs and slowly swam over to it. Using all his strength, he managed to drag both himself and Kate up the ladder using only one arm, the other clutching her tight to his side. Exhausted, he flopped down on the dock with her landing on top of him, still unconscious. He rolled them over to examine her, putting his head against her chest to listen for breathing. His trembling fingers found a place on her neck, feeling for a pulse. Castle closed his eyes, concentrating, praying, that she was still with him. After a few seconds, he still felt nothing.
He immediately began chest compressions, willing her to stay alive.
"C'mon Kate, do not die on me. This is not how it ends, not how we end."

He leaned down, tilting her head up and plugging her nose, filling her lungs with the air she could not take in herself. As her lungs expanded and filled with his air, he prayed she would be okay. Losing her was not an option. Kate Beckett was not someone you could get over losing.
Finally, she began to cough, expelling the murky water from her burning lungs. Castle gently rolled her to the side and rubbed her back, whispering comforting words into her ear.
When the coughing had subsided, Castle gently pulled her up into his arms, cradling her shivering body next to his.
"C-Castle?" She said weakly.
"I'm here. I'm here." He said, exhaling a shaky breath into her hair.
"Car...water..." Kate said, the confusion clouding her head.
"Y-You're okay, I got you- got you out. I had to use your gun to shoot off the belt and break the window." Castle said, his teeth chattering quietly next to her head.
"S-so cold." Kate said, her head burrowing into his chest.
He wrapped his arms tighter around her small body, willing the little warmth he had unto her. "Is s-someone coming?"

"Boys will f-figure it out eventually, call it in." He said.
Kate nodded against his chest and closed her eyes, her shivering body filled with exhaustion.
Castle gently stroked her hair, resting his chin against the top of her head.

After a few minutes, Kate raised her head to look at Castle.
"Rick." She whispered.
"I'm here."
"T-Thank you…for getting me out."
He smiled and out of a brief feeling of confidence, leaned down and kissed her forehead gently, his lips lingering.
"Always."

She hugged him tighter, burrowing her face into his neck.