It was a stupendously ridiculous affair, looking back. After all, John Watson had been born on an island, surrounded by ocean. Lived in a city that depended on its waterways for its lifeblood. Had served in the army - presumably travelling to battles via sea and sail, and had so many other useful, practical skills that it simply never occurred to me that he was unable to swim.

I had not been overly concerned when a lucky blow in the heat of battle had knocked him overboard. A few stout blows of my own had quickly dealt with the miscreants we had been pursuing - a rather sordid smuggling affair that was, upon reflection, completely unworthy of our attentions - and I leaned over the edge of the boat, fully expecting a somewhat sheepish waterlogged figure peering back up at me in chagrin.

When nothing but the dark waters of the Thames met my gaze, I suddenly realised that the fanciful phrasing of one's blood 'turning to ice' was in fact horrifically apt.

Hope surged once, blessedly warm as a familiar head broke the surface of the water only to turn back to abject terror when I realised that Watson was not in fact swimming, but was drowning. The dully panicked eyes met mine once, then slipped beneath the oily waters with a resignation that was almost sickening.

Taking off my coat, my jacket, it all required time I didn't have, but with a clarity of thought that seemed almost inhuman, even to myself, I knew the added weight would only impede my own swimming efforts. My shirtsleeve snagged on an outflung nail and I tore it away completely in a motion that would no doubt have given poor Watson a conniption, no longer giving a damn about the condition of my clothes, the chill of the air, or the groggy thugs rousing behind me as I wrenched at my collar, mounted a nearby coil of rope and dove into the water.

The Thames is not an ideal swimming-hole in the best of times; the ordure and effluence of the city seep constantly into its waters despite the recent waterworks, the runoff from boats and rubbish from the streets cycling endlessly through its veins. In the dead of night it takes on a quality that is almost terrifying – no visibility at all to be had, no frame of reference, nothing but the water and the feel of my own pulse beating thickly in my ears.

Of Watson there was no sign whatsoever.

I redoubled my efforts, folding my body like a jack-knife and dove deeper. I strove to be methodical, to retain my mind but I fear I was fast losing my grip as the pressure around me seemed to build and my searching arms came up empty again and again. I rose to the surface under the demand of my own lungs only to snatch a hasty gasp of air before diving again, part of my mind relentlessly counting the seconds as they eked into minutes and on into aeons.

How long could a man survive without air? For once my formidable memory failed me utterly.

Mercifully my hand at last snagged on a familiar worsted collar, the weight beneath it telling me I had achieved my goal at long last. I struck for the surface, dragging his appallingly still weight behind me. Holding him to my chest I made for the shore, the dirt and stones cutting my shins and knees as I stumbled under his weight. He wasn't breathing. he wasn't breathing and although I have found that panic is an emotion of no use whatsoever I could not prevent it welling up in my chest.

His heart still beat under my trembling fingers and I tore at his shirt, his collar, laying him out amongst the detritus of the tides. I pressed his knees towards his chest, pumping his arms, trying to re-start the great engine of his lungs. I was dimly aware of words escaping my lips as I worked, but I cannot recall their exact nature. "Please," I know for certain I had uttered, many, many times. God. John.

His chest still drew no air, and the pulse of life was flickering ever-fainter at his throat. In desperation I closed my lips over his, pushing air into his lungs from my own as if somehow I could breathe for us both. Once, twice, and then again. They say that madness is the repetition of the same act again and again, desiring different outcomes, but the truth is I was mad with terror already.

I am lost without my Boswell. It is a trite saying, but trite lies in truth, and the truth is that a life without John Watson by my side is no life at all.

And then, finally, he breathed.

A gout of water erupted from his lips and I hastened to support him, turning his head as he expelled great quantities of the foul water in sharp, hacking coughs. He took in another breath, and then another, and I pressed my hand to his forehead, pulling him against me, needing to feel the motion of his lungs.

Regrettably, my relief turned to anger, as it so often lamentably does. For one to feel relief, one must have been in a position of peril; for one to have been in a position of peril, one must have been placed that way through an action of unforgivable rampant stupidity. As Watson hunched over another series of coughs, half-gasping my name I grabbed his shoulders and shook him sharply, demanding to know what he was thinking, how he could have been so stupid, a series of ridiculous accusations and remonstrations against his character.

I must have been mad. The poor fellow was half-drowned, coughing and shivering in the cold night air, covered with god knew what humours from the foul waters and I was shaking him, shaking him so hard his head was almost snapping over his shoulders.

Abruptly I came back to myself, horrified by my own actions. "Forgive me," I crushed him to my chest, wrapping my arms tightly about him, afraid that even then he would be snatched from me. "Forgive me, my dear Watson. Forgive me."

His hands came up clumsily to grasp at my arm across his chest, looping around and clutching at it as if I were a lifeline. He gasped something that could have been my name, one hand groping at my knee and tapping it in graceless, patting motions of wordless comfort and I pressed the side of my face to his hair, feeling the water slick my face and mingle with the tears there.

Even in this extremity, shaking and coughing in my arms he sought to comfort me. I shall never know the measure of Watson; the strength of his character and the size of his heart are far more than I will ever deserve.

But one thing was certain: we would begin taking swimming lessons in the future. Immediately.