Bruce's aim is to become the youngest person to make the 14-kilometre annual swim from Cape Tormentine, N.B., to Borden-Carleton, P.E.I.
But she says her greater goal is to honour her cousin Kevin Laffin, who died at the age of 15 from brain cancer.
Bruce, a specialist in long-distance racing, signed up for the Big Swim after hearing donations from the event go to Brigadoon Village, a summer camp for young people with chronic illnesses.
"When I found out about the Big Swim and that it was for kids who were cancer survivors, I knew I wanted to do it," she said in an interview from her home in Truro, N.S.
"Before he died, Kevin always wanted to go to camp, any camp, but they wouldn't let him because of his illness," she said.
"I can let kids be normal at camp."
Each $1,000 raised from the crossing can send a child to Brigadoon Village, where young people camp for a week at the picturesque facility in Nova Scotia's Annapolis Valley.
In addition to setting a record based on her age, Bruce is also trying to beat the female record of three hours and 45 minutes for the distance.
"If Cali keeps to her plan and keeps to her goal, she'll break that," said Colin Bruce, Cali's father and coach, adding that her time will depend on water conditions on Sunday.
The swim can stretch into 17 kilometres as currents push swimmers back and forth along the route near the Confederation Bridge, lengthening their time in the water.
Bruce said his daughter trains by doing 20 repeats of a 50-metre swim — two laps of a pool — followed by a one-minute rest, and then a repetition of that pattern.
"It's an attack process. She swims a kilometre, has quick bite and something to drink and off she goes," he said.
Bruce's pace is kept on track by a kayaker who paddles alongside her and slaps the water with her hand when a kilometre has passed.
The water is cold, but swimmers wear flexible wetsuits and tight-fitting booties that keep their bodies warm.
The swim across the Northumberland Strait has grown in popularity in recent years, with 49 swimmers signing up this year.
Each participant raises at least $1,000 for Brigadoon.
Kerri Ann Hillier, the camp's fund development officer, said this year swimmers had raised more than $270,000 as of Wednesday.
Hillier, who is swimming the event for the first time, said it has evolved into a key fundraiser for the camp over the past four years.
Participants of all ages and varying speeds leave in groups, with some taking up to eight hours to make the crossing. They arrive at a small beach to sprays of champagne and hugs from friends and relatives.
Cali Bruce said if she feels fatigue along the way she'll recall happy scenes of early childhood, including a moment when she learned to swim in a cove off Cape Breton.
The boy who held her as she took her first strokes in the icy East Coast water was the cousin she's honouring on Sunday.
"He'd take me down to the ocean before he got sick," she said. "He was the first person to get me in the ocean."