A smiling, upbeat McCardel arrived in the morning at a rocky jetty in Havana's Hemingway Marina in a pink 1950s Chevy convertible. She carefully adjusted her black swim cap and goggles while her husband applied grease around the edges of her suit to prevent chafing.
"As confident as I can be. I think it's all going to work out well," the 28-year-old said. "It'll be tough, though. It's not going to be an easy ride, but we'll get through it as a team."
McCardel expects to take about 60 hours to get to the Florida Keys, a little more than 100 miles (160 kilometers) to the northeast, braving sharks and jellyfish along the way.
The current weather report for that window is clear. The sea off Havana was flat and glassy Wednesday, precisely the ideal conditions that McCardel's science team had forecast.
The Florida Strait has been busy the last three summers, with fellow marathon swimmers Diana Nyad and Penny Palfrey making four failed attempts at the crossing between them since 2011. Australian Susie Maroney successfully made the swim in 1997, though she did it with the benefit of a shark cage.
"It is the hardest swim in the world today," McCardel said earlier at a news conference in the Cuban capital. "No one has been able to achieve this. It's possibly harder than winning the World Cup or getting a gold medal."
The challenge also outstrips by far, at least in terms of distance, anything she's done before. McCardel, who has twice made a double crossing of the English Channel, said the most time she's spent in the water continuously is 25 hours.
She is swimming under English Channel Marathon rules, which means she cannot touch her support boat or hold onto anything. Nor can she wear a full-body wetsuit, which would help protect against exposure and jellyfish stings, or use a shark cage.
A piece of equipment called a Shark Shield creates an electromagnetic field around her in the water, discouraging the predators from getting too close.
McCardel plans to stop every half-hour or so to sip an energy drink, preferring that to solid foods.
She and her team have spent the last nine and a half months planning the trip and studying others' attempts to try to figure out why they were unable to complete the swim.
They picked June for their bid in a bit of a tradeoff: While seas are warmer later in the summer, this month typically sees lower concentrations of box jellyfish, whose dangerous stings have scuttled past attempts.
They even took the lunar cycle into account. Moonlight attracts jellyfish to the surface, and that should be less of a problem as she sets off under a new moon.
McCardel said she believes she can succeed where others fell short because she has assembled an unprecedented team that includes scientists on land who are experts on the Gulf Stream current that flows through the straits.
They will be crunching data in real time and feeding information to her support boat, a 44-foot catamaran dubbed the "Sunluver," so the mission can dodge things such as the powerful eddies that have swept others off course.
"The advantage that this gives us is that we can foresee 10, 20, 30 kilometers ahead," she said. "So if we can slightly change our course to avoid things in the future, we're less likely to get picked up by an eddy off the Gulf Stream and pushed in the wrong direction."
Still, she acknowledged there's no way to guarantee nature's cooperation.
"The Gulf Stream ... it's like a wild animal," McCardel said. "You cannot predict it that much in advance, so you cannot take historical data from Penny Palfrey or Diana Nyad's swim and say, well, this is what happened to them, therefore if we don't do exactly the same then we'll have a better outcome."