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John didn't want to go to the seashore. He didn't want to share a room with Harry, he didn't want to sleep in the sandy bed that smelled like salt, and he most definitely did not want to wear the swim trunks. They bogged him down and made swimming, the fun part of being at the beach, difficult and cumbersome. 

However, his parents didn't listen, and soon he was being bundled out the door and into the tiny car to fight with Harry over the window seat and stare at the headrest in front of him in boredom.

Two hours later, they reached the beach house and he raced out of the car, determined to get as far away from Harry- who'd begun to call him names about twenty minutes ago- as possible.

Ignoring his parents' calls for him to come and help unpack the car, he scrambled down the rocky beach to the shore, digging his bare toes hesitantly into the sand as he meandered down the shore. 

This was John's private beach. He never told anyone about it, and it was the only part of being at the seashore for holiday that he liked. Here, there were no people, and when he swam, he swam naked or in just his pants. It drove his parents crazy, but sometimes he thought it kept him sane, to not have to be in the tiny rented beach house with everyone else and swim in public.

Just now, though, he knew he'd get in trouble if he stayed long on the shore, so he tucked a shell into his pocket and regretfully scaled the rocks again and went to rejoin his family. He didn't see the tail disappearing beneath the waves as he left.
**
Later, when they'd settled in and had lunch, he went back down to his beach and shucked off his clothes. He could hear Harry shrieking distantly as the cold water lapped at her toes, and shook his head at what a baby five year-olds were. As a seven year-old, he was an excellent judge of such things.

He didn't hesitate to wade into the water, occasionally turning back to chuck a shell or stone towards the shore to collect again on his way back. He floated for awhile after that, and was just about to dry off when he saw him. A man, dark hair and pale skin and green eyes, just- watching him from beneath the waves.

Forgetting, as children are wont to do, the rules about strangers his parents had drilled into him for years, he moved back towards the surf, hands twisted in front of him. After a moment, he dropped his hands, and watched as the man hauled himself onto dry land. It was a mermaid! John thought of those silly movies Harry liked, the ones about a red-headed mermaid who liked to sing, and shook his head.

This one was more exciting, and he was slouching on the rock, staring at John curiously. He beckoned, and John grinned and hopped across the rocks and into the surf until he stood before him. The mermaid- merman?- cocked his head, then poked John's knee curiously. 

He considered him again, then made a series of strange clicking noises. "What?" John asked blankly, blinking at him. The man tugged him close, so their faces were inches apart, and clicked again, pointing at the sea. "I can't swim that well," John protested. The man made an impatient noise and abruptly changed topics, poking at the red fabric of John's pants where it slipped on his hip. 

After a moment, he shook his head and looked up again, tilting his head at John. He patted John's hands, than his own shoulders and back, then John's feet. He gestured to himself again, and John, catching his drift, clambered onto his back, hugging him tight to keep from falling off. With some wriggling and then one strong kick, they slipped beneath the waves.

**

At low tide, after hours of searching, Olivia Watson found her son's trousers, tied tightly around his bundled shirt. Three weeks later, John was pronounced lost at sea. Harry never did recover.

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A few minutes later, they were several miles out from the shore. Sherlock didn't realize anything was wrong until he felt the land-boy's limbs slacken. Did he want to go back? He glanced over his shoulder at the boy and clicked questioningly, but the boy's eyes were closed, and- he wasn't supposed to be blue, of that Sherlock was sure. Panicked, he kicked his tail hard and broke through the surface of the water, gills on either side of his neck automatically sealing themselves against the harsh dry air.

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John hadn't realized the merman was going to take him that deep underwater, and soon the water overwhelmed him. He choked on the water rushing into his nose and mouth, watching longingly the air-bubbles which rose from him. As his grip began to loosen on the shoulders of the creature who towed him through the water, he had a fleeting thought that maybe this way, he'd at least get some air again. The iron bands around his lungs tightened, and he slipped into darkness.
The next he knew, there was a sharp pain on either side of his neck, and then the pain of salt on raw tissue- and then the pain faded as oxygen flooded his system and he found the energy to expel the water from his lungs.
The merman was staring at him in obvious shock and concern, one bloody fingernail held aloft. When he realized that John was watching him, he absently pressed the blood onto the boy's wet lips- didn't need sharks around- and slipped with him back beneath the waves.