Call me when you get home! Andrea said when she dropped off the picnic basket at Tess’s house that morning.

Okay, I will! Tess said. She was wearing a red bikini and jean shorts, and a pair of red sunglasses with white polka dots. In a lower, more serious voice, she said, Thank you for the picnic. You didn’t have to go to all this trouble.

I know I didn’t, Andrea said. But I wanted to.

Andrea and Tess were first cousins. Their fathers were brothers, the mighty and formidable DiRosa brothers, both narcotics detectives for the Boston Police Department. Andrea had three younger brothers and Tess had three older brothers, so in addition to being cousins, they were sisters, the only DiRosa girls. Andrea was nine years older than Tess, so in addition to being cousins and sisters, they were mother and daughter.

In all of Andrea’s memories of childhood, she was wet. She was diving into the community pool at the YMCA, she was swimming fifty yards off the shore of Thompson Island, sluicing through the green water of Boston Harbor; she was a mermaid, the water was her natural habitat, it was hers and hers alone. Her brothers fooled around in the waves at the beach, but they didn’t swim the way Andrea did.

Andrea’s natural ability in the water gave her authority. Because she was so gifted at something, adults trusted her, they treated her like she was older; from the time she was twelve years old, she was allowed to baby-sit for Tess; she was allowed to take Tess for long walks in her stroller. When Andrea was fifteen and received her junior lifesaving certificate, she was allowed to lead beach excursions for her brothers and her cousins—eight DiRosa children, with Andrea in charge. Andrea swam butterfly for the Boston Latin swim team, and in her senior year she won the city championships in the 100-meter and 200-meter. That same year she pulled a grown man who had had a heart attack while swimming laps out of the middle of the pool at the Y. She was awarded a medal by the city council; her picture was in the Globe.

When Andrea was eighteen, she got a job as a lifeguard at L Street Beach. She wore a red tank suit and zinc oxide on her nose. She had Ray-Ban aviator sunglasses and a whistle on the end of a braided cord that she spun around her fingers, first this way, then that way. When Tess came to the beach, she sat, quite literally, in Andrea’s shadow, on the first rung of the lifeguard stand, sucking on cherry Popsicles. In Andrea’s memory, it seemed like Tess was there every day. Andrea would look down and see the straight white part in Tess’s dark hair. Tess would swim, and Andrea would watch her. Tess would attempt the butterfly, but she flailed and humped. Andrea tried to teach her the movement, but Tess’s shoulders weren’t strong enough yet, and Andrea was busy working. There were hundreds of kids to watch.

It was at the end of the summer, the week before Andrea started her freshman year at Boston College, that Tess nearly drowned. Andrea was spinning her whistle, scanning the sand and the shallow water, dreaming about finally living away. After months of battling, her parents had agreed to allow her to board, although she easily could have commuted. In the end Andrea’s winning argument was that she was attending BC on a partial swimming scholarship and the team practices were at dawn. You don’t want me riding the T to Brookline at four in the morning, do you?

In truth, Andrea wanted to separate herself from her family. The DiRosa clan was too close-knit, too loud, too steeped in the politics of the BPD, too Italian, with their garlic and ricotta and veal involtini, their heavy gold crosses and crucifixes everywhere she turned. Andrea wanted to experience a life that was quieter, more reserved, more refined, a collegiate life, an intellectual life. (Lord help you! her grandmother said. You’re going to the Jesuits!) The fact that Andrea had not been able to escape the city of her birth discouraged her a little, but there had been no avoiding a Catholic education if her father was to pay for it. Andrea had gotten into Notre Dame as well, but South Bend was deemed too far away.

Andrea was thinking these things, she was twirling her whistle and scanning the shallow water, she was listening to Van Halen on someone’s boom box, she was enjoying the sun on her shoulders, when the feeling struck her: a panic like a sickness. It was as though she had looked down and noticed the lifeguard stand was gone, she was sitting on thin air, about to fall. It was Tess who was gone. This was not unusual—Tess swam and played and visited the snack bar and the restroom just like everybody else. Andrea saw Tess’s Popsicle stick, stained pink, sticking out of the sand. This wasn’t unusual either; Tess, at age nine, was a habitual litterbug.

Lifeguarding was a job that required assiduousness rather than instinct, but it was instinct that kicked in. Andrea scanned the water out past where any other nine-year-old would be swimming, and there she saw a hand. Or what she thought was a hand. A hand!

Andrea blew her whistle—three short blasts, an emergency! She jumped recklessly from the top of the stand and nearly broke both her knees. She grabbed her board, dashed into the water, and started paddling. Andrea spied a flash of someone’s face—yes, it was Tess! Tess was out way over her head. What was she doing out there? The face disappeared, the hand slapped the water. Andrea abandoned her board; it was slowing her down. She was the fastest flyer in the city—she could get there quicker on her own. She swam to the spot where the hand and face had been and dove down and pulled Tess up off the bottom. The effort of this, of getting sixty pounds of deadweight to the surface, nearly killed her. Tess was waterlogged. But not dead, right? Andrea could not let herself worry about anything except textbook lifesaving. Get the swimmer under the chin and paddle with her to the board, secure the board under Tess, and swim for shore. There were lifeguards coming toward her, three of the big lunks Andrea worked with, whom she had thought completely useless until this moment. One of them, Hugo, took Tess and the board and powered her to shore. The other two guards, Roxbury and Toxic Moxie (these were their nicknames; Andrea had no idea what their real names were), laid Tess out on a towel and pumped her chest and gave her mouth-to-mouth while Andrea stood at Tess’s feet and shivered and said the Hail Mary and promised God that if Tess lived, Andrea would repay him by becoming a nun.