When you're in the middle of an ocean and your boat is on fire, it's probably time to panic.

Julia Immonen had gone from novice rower to crossing the Atlantic Ocean in 18 months all in the name of raising awareness about modern day slavery.

Fifteen days into a 45-day journey, their water maker blew up and caught fire. It was just one of many challenges which caused her to rely on God like never before.

In 2012 Immonen was part of the first female five-person crew to cross any ocean, as well as beating the world record for fastest female crew crossing the Atlantic. Her new book, Row for Freedom (Thomas Nelson), charts the story of getting into the boat and surviving the 3000-mile journey.

It all began in June 2010 when, at the end of a jog, her flatmate casually suggested rowing the Atlantic. Immonen was hooked.

"We were running along the Thames and she just said 'Julia, shall we row the Atlantic?' and I was like 'Yes' as if it was like grabbing a coffee from Starbucks – not quite knowing that I'd signed myself up for what is known as the world's toughest rowing race," she says.

Even though her friend had to pull out, Immonen felt that it was a calling from God. Someone who didn't know her situation had prayed for her and said she saw a picture of her in a boat with authority over the storms. Immonen also felt the significance of rowing a route that was the major crossing for ships carrying slaves in the 19^th century.

She had already been captured by the horror of human trafficking. Watching the film Taken, followed by hearing about the reality of the story in that film, started her on a journey of researching and lobbying about a global issue that's frighteningly close to home.

Human trafficking is the world's fastest growing crime, affecting an estimated 30 million people worldwide. The movement of people for labour and sex is also big business, thought to have a market value of $32 billion. Recent cases in the UK have highlighted the underground problem at home – something that will be present in at least some of the 300 brothels that are in one London borough alone.

Immonen's own experience of vulnerability gave her some insight into how easily it can happen. "I guess at that really vulnerable age of 15 or 16, I looked for what I should have got from my father in relationships with men," she says.

"There are people who are so much more vulnerable because they've grown up in poverty and they're the ones being groomed. I guess that just resonated for me – it's not hard to be groomed by a guy under false pretences or under false promises of hope."

This sense of purpose kept her going. There were enormous obstacles to getting in the boat: after her friend dropped out, the second crew she joined also fell through and then her mother became ill. And then there was the physical challenge itself: seasickness, sleep deprivation, pain, fear, not to mention having to go to the toilet in a bucket in 50ft waves.

"My pain would pale into insignificance when I thought about the millions of men, women and children being exploited; women having to service 30 or 40 men a night. I had the freedom to get off the boat if we made it to Barbados, but there are people who don't have that freedom.

"I am just so passionate about this. I thought 'What's the worst that can happen? I'll die in the ocean and people will hear about human trafficking.' I was prepared to do whatever it took. But I think that's a God-given passion."

The physical challenge had a spiritual dimension too. "When you're pushing back spiritual darkness, which this is, then it's going to be hard," she says. "People hear the glory and the victory of the row, but they haven't heard the private battle that goes alongside that.

"I felt the spiritual attack was thick. I knew that some days it was the prayers of thousands of people that carried us. Our boat should not have got across to Barbados; it had so many holes in it, it wasn't even really seaworthy, everything that could go wrong did go wrong. It was literally against all odds."

Her motivation is clear, but what does spiritual life look like when reading the Bible is rather impractical? "Kate, the other Christian in the boat, would give me a sermon title at two in the morning when we were ready to give up and I would preach everything that I knew about faith, about joy, about fear; I quoted every flipping Bible verse that I knew, I declared every promise out loud, regardless of if I felt like it or not.

"My faith soared. It was a 24/7 conversation with God. I clung on to the Lord in a way that I have never done. I missed the boat as soon as I got off because of that intimacy with God."

And what about now? Two years on, what difference has it made to life back in London? "I remember driving my car for the first time and this lady starting having this road rage with me. And I thought 'Buddy, go row the Atlantic and you won't be road raging like that.' I feel like I've been given this perspective and patience.

"I've had to depend on God and I've seen him come through for me. I feel like I've been given the crown jewels."

• Julia Immonen is now CEO of Sport for Freedom, a charity that uses sport in the battle against human trafficking. The row raised £100,000 for the charity, as well as raising the profile of modern day slavery.
• Her next journey involves cycling around the UK, visiting five major cities where the historic slave trade was particularly prevalent – starting in Liverpool and going through Birmingham, Bristol, Oxford, finishing in London. Sign up here to cycle the final leg of the journey in London with Sport for Freedom on October 18.
• Row for Freedom is out September 2.