Magic realism was the topic under discussion.

Surely, an iguana would never topple from a tree-top in the garden of a white woman – even in Trinidad, suggested an audience member to Monique Roffey at a recent literary festival in Devon.

"Only in the Caribbean," replied the acclaimed writer, going on to explain why such events are daily occurrences in Trinidad, the island where she was born and bred – but never in more temperate climes such as Britain, of course. Then the audience burst into laughter. "I thought it was because I was being witty," jokes Roffey. What she had not seen was the large ginger cat, tail erect, strolling behind her at a stately pace.

"Suddenly, it leapt onto my lap and it stayed there, sleeping, throughout the event," says Roffey, in whose thoughtful, engrossing new novel, Archipelago, the tale of the falling iguana is just one of many compelling stories related to mesmerising effect.

So who knows what might happen when Roffey comes to the Edinburgh International Book Festival? As she writes in Archipelago, which is about climate catastrophe as well as father-daughter relationships: "Animals have a keen sense of time." Perhaps an old dog like Archipelago's Suzy, whose "bull terrier's nose is long and Roman, a pink patch at the end like a piece of a ballet shoe, worn satin", will mimic that cat.

Suzy plays an important role in the novel. Her owner, Gavin Weald, voyages with her and his engaging six-year-old daughter, Océan, across the open, turquoise Caribbean Sea, through the Panama Canal, into the Pacific to the Galapagos Islands, leaving a sea of troubles behind for far more treacherous, unfathomable, uncharted territories, both physical and emotional. There are enchantments too: fishes that fly, affable seals that seem openly amused by humans, and a great white whale. "My nod to Moby-Dick, a favourite book since childhood," admits Roffey.

When her novel begins, it's only 12 days since Gavin and Océan have moved back into their once-comfortable, middle-class Port of Spain home, which has been devastated by cataclysmic floods. Following a family tragedy, the pair are drowning in grief.

Middle-aged, flabby and unfit, Gavin walks out of his job, stocks up on supplies, ditches his mobile phone and sets sail, with Océan and Suzy, along the coast of Venezuela and Colombia, in his old-fashioned, long-abandoned boat, the Romany, which he handled effortlessly when he was a muscular young man with an experienced crewmate. It is a fraught voyage as deadly currents, tropical storms, a violent robber, sharks, doldrums and – more scarily – personal demons are battled.

The daughter of a half-Egyptian-French mother and an English father, Roffey is a tanned, voluptuous 47-year-old with a tumble of sun-streaked blonde hair. She was Orange Prize short-listed for her wonderful second novel, The White Woman On The Green Bicycle, about the liberation politics of Trinidad, and is never out of her depth in the dangerous waters she dives into in Archipelago. And that, she reveals, over tea in the offices of her London publishers, is because she made a similar boat trip between December 2010 and April 2011.

She has the scars to prove it. "Would you like to see my scar?" she asks, rolling up her jeans, revealing an island of puckered pink skin on her calf, on which she thinks she will eventually have a fish tattooed, or perhaps a mermaid – as this is how Gavin describes his adored, silvery-haired daughter.

Opening a copy of her novel at the end, where Romany's voyage is mapped, Roffey traces her journey. "I did it mostly by boat, west out of Port of Spain, Trinidad, towards the Panama Canal." She's a strong swimmer but discovered that she's a rotten sailor when they took to the open seas. She suffered wretched sea-sickness – as do Océan and Suzy. Recovered, she island-hopped until she fell out of bed when the boat was navigating storm-tossed seas, slicing open her leg. "Obviously, I was unable to sail to the Galapagos Islands; I flew there instead."

There is, though, a profoundly personal story underlying Archipelago. About four years ago, Roffey's older brother Nigel's home in Maraval, Trinidad, was badly flooded. "It rained and rained during the day; then at night it rained again. Water poured down the deforested hill – his house sits at the bottom. The house wasn't demolished but it was ruined. The water was chest-high, catastrophic. His two small daughters were there, although his son was not. The housekeeper almost drowned; the dog was swept away.

"My brother and I lead very different lives. I have no responsibilities, although I am about to buy my first flat in London. Nigel's a good man, a family man, with a good job; it's a simple life that should be safe to live. As I stood outside his house with him the next morning I thought, 'OK, here it is – climate change. It's right here in my family.'"

Clearing up was like an instantaneous war effort for the neighbourhood, says Roffey, who has been a centre director for the Arvon Foundation, a UK-based charity which promotes creative writing, and held posts with the Royal Literary Fund at Sussex, Chichester and Greenwich universities. "We all just started digging."

Days later she began to take photographs and make notes, because she knew she would write about it. Although this was the starting point for her novel, "the rest is pure fiction". The more she thought about how she could write about how we survive in the world amid climate change, the more she worried about our levels of denial and that people have become bored with this, perhaps the biggest moral issue of our time.

"We all know the planet is melting along with the banks," she sighs. "But it's not a sexy subject any more. People just aren't that interested. I mean, who cares, apart from me, my brother, our family and others similarly affected by such disasters? What I really wanted to show is how a good life like my brother's is no longer a safe bet, although he's since fortified the house and a reservoir has been built."

She spent a year thinking about the novel, then she remembered her brother's old GD28 boat, Romany, on which they had had a lifetime of coastal trips and sea adventures. He'd sold it years ago and now it's rotting on its moorings. "I asked him where would he have liked to sail in Romany, what was his young man's dream? He replied, 'The Galapagos'. He'd never mentioned the islands before. But I had my novel, which I knew would be about what happens after a flood. So Archipelago is the simple story of a journey. Maybe it's too simple, maybe it's not political enough, but I always knew that I had to take that journey myself."

Roffey is part of the proud new wave of young post-colonial Caribbean writers, "especially young women", following in the wake of VS Naipaul and Derek Walcott. Indeed, her next novel will be a big one, set in the Caribbean, her heart's home. It will be an adventure to embark upon the writing, she says.

Summering in London and wintering in Trinidad, Roffey is no stranger to daring voyages of discovery. After the break-up of her long, largely celibate, relationship with British writer Ian Marchant, she set off on a "sexual odyssey" which she details in her erotic memoir, With The Kisses Of His Mouth (2011), a sort of real-life but well-written Fifty Shades Of Grey, in which she discloses how she started having sex with strangers – from sex clubs in French nudist resorts to tantric sex workshops in country house hotels in Somerset.

Her mother, now aged 80, who proudly displays her daughter's literary output on her shelves at home in Trinidad, has hidden it in a cupboard, although Roffey insists she's no prude. Her glamorous parents – her father died 20 years ago – had "a very sexy marriage. They were always lovers, so I suppose I wanted that sexiness in my own life." When she sent a copy of her book to her ex, he said: "I hope it sells a million copies and that you never live to regret it."

Does she regret it? She looks shocked at the naivety of the question.

"No, no, no! I was in my mid-40s when I wrote it; it's a story about a mature person taking mature risks. Of course it was a big risk to write it. How would it affect my career as a literary novelist? But I felt I had something to say, my own story to tell. It wasn't a reckless decision. Anyway, it's my job to take risks – I plan to take many more."

Archipelago by Monique Roffey is published by Simon & Schuster, £12.99. Roffey appears with novelist Morag Joss at the Edinburgh International Book Festival on August 15. Go to www.edbookfest., or call 0845 373 5888