Everything hurts. It's dark and noisy and everything hurts, and Sherlock gasps in a breath but it's water, and panic and pain, everything hurts and it's frightening, and Sherlock is almost never frightened of anything but in the last fifteen minutes he's been abjectly terrified more times than he's ever been in his life, and now the water is filling his lungs and he can't see or hear and something pushes him, a strong hard shove, and it isn't until his head breaks the surface that he realizes it was pushing him up, and he gulps down air and can't breathe, still can't breathe, and a hand pulls him up, out of the water, he can feel broken bones and tears in his skin react to the heat - why is it so hot, he was at the pool, it was chilly there - and he vomits, chlorine-treated water and his own bile and coughing, just enough presence of mind to turn his head so he doesn't aspirate it, his throat is raw and he croaks out a name before he passes out, "John?" and then he is swallowed in darkness, a different kind than before, quiet and cold.
When it's bright again, Sherlock is lying on a hospital bed. His brain is working, but not well enough, so he knows he must be drugged, which means burns and probably internal bleeding. He keeps his eyes shut - too bright, too much light just yet - and catalogs memories. He remembers going to the pool, he remembers the shock and the drop of his stomach at that first glimpse of John, he remembers having to hold himself back when John revealed the Semtex vest. He remembers dancing red sniper lights, he remembers a slamming door, he remembers...He shudders. He remembers Moriarty, his voice (Dublin, lower-class but educated, playing with intonation and careful word choice, intelligent, psychopath), his teasing, his utter disregard for John. He remembers, and makes a mental note to give Mycroft everything because if Moriarty hurt John then Sherlock will hurt Moriarty. And then it flickers in his memory: he remembers holding the gun, he remembers John giving a jerk of the head to signal his understanding, he remembers Moriarty grinning with self-assurance that Sherlock wouldn't do it.
He doesn't remember anything else until someone pulled him out of the pool. Now is the time to open his eyes, to see John sitting beside his bed, probably asleep (it's been a long day for him, Sherlock will forgive it), see Mycroft's assistant texting status updates, see Lestrade waiting for the description Sherlock can give which will be more useful than the one John already gave. Sherlock prepares himself.
When he opens his eyes, he thinks, he will tell John that they need to have a discussion about the events of the day. That the heart Moriarty spoke of is obviously a metaphor, not his literal heart (which appears to functioning relatively well if the beeps of the monitor and his own observations are to be trusted, which they are, of course), and that this experience has proven to Sherlock that he needs John around, needs him not to leave, not ever. Sherlock knows that John is quietly bisexual, knows that John finds him attractive. Sherlock isn't very sexually motivated, but he needs John, needs him in both a literal (John is helpful, has many useful skills) sense and a more abstract (John makes him feel happy and safe and appreciated all the time and no one makes him feel things other than frustration, and he's gotten rather used to feeling other emotions and if John is gone he won't) way.
He opens his eyes. John is not in the chair beside his bed. John has not been there - no half-read shoddy magazine, no depressions in the chair to match his frame, no residual warmth or scent to show he'd simply popped out for the toilet or a drink. John isn't there. Sherlock panics again, his heart racing, his voice crying out for John; nurses arrive, stick him with something (a sedative, obviously, he hates sedatives with a passion, hates the way they slow his mind even more, don't the stupid nurses understand he needs his mind, he needs it to find John), and the darkness swallows him up again.