“She’ll be fine,” his wife mumbles from the edges of sleep.

“All right.” He waves her off.

Finn watches wistfully as his big sister vanishes. He was hoping she would stay and dig the hole with him. He can already see that their dad was just pretending to be interested, and is about to leave him to get on with it “just for five minutes while I sit down”. His mother won’t want to get sand in her tanning-oil. Now he’s stuck by himself with a big circle of sand to dig out.

If Ava was helping, she’d already have taken over. They’d be knee-deep beneath the surface, and he would have been assigned a specific menial task, such as building a surrounding wall to make the hole even deeper. While he periodically rebels against her bossiness, just so she doesn’t get the idea she’s in charge of him, he also secretly likes it. It’s comforting to be told what to do.

Besides, however much he complains, Ava is in charge. She knows more than him. She can run faster than him. She can read harder books than him. She can swim in the ocean, breaking past the surf to slip between the wave’s peaks like a mermaid, until he’s afraid she’ll swim on and on into the horizon. She can do anything. She is the person he admires the most in the whole world.

The only thing he doesn’t understand is why she’s so bad at managing their parents.

“This hole’s going to be really great,” Finn tells his dad, now back on the lilo.

“That’s good.”

“It might need decorating. To make it look even cooler.”

“Ava’s getting some seaweed.”

“She might not get enough, though. It’s going to be a really big hole. With a wall.”

“A wall? That’s nice.”

“Yes. A really, really big wall.”

“That sounds good.”

“So, do you think I should get some more seaweed? Just in case she doesn’t get enough?”

“Can you find your way back all right?”

“Easy. I just look for our windbreak.”

His father smiles affectionately. “That’s my boy.”

The drowning begins gently. First, the shallows, seductive sheets of flat sun-warmed water. Then, the smallest breakers, little rolling waves that barely wet his knees, enticing him in further.

The swim in the pool was a test, a final experiment. He will join his big sister in the water, swim up behind her and say, ‘Oh, hello,’ calm and casual as if he’s done this every summer of their lives, and she’ll be even more surprised and impressed than when she saw him in the pool. He won’t be stuck on the shore while she disappears off on her mysterious water-journeys. He’ll be able to keep up with her at last. He’s done this a thousand times in his dreams.

He rehearses how she does it in his mind. She wades out, gasping, until the water’s up to her chest. Then she turns her back and lets a wave wash over her shoulders. Then she turns around again and starts swimming, and there’s a moment when she stops gasping and starts laughing, as the cold holds her so tightly she doesn’t feel it any more, and she disappears and reappears, disappears and reappears, until eventually she’s glowing and filled with savage life.