THE ocean is a ruthless killer. “It's so much more powerful than human beings and always has been,” says Charlotte Runcie, who is standing on Portobello Beach looking out to sea. “Storms can make mincemeat of humans in boats and can completely destroy seaside communities.”

For evidence, you only have to travel 50 miles down the coast from here to Eyemouth, where the worst fishing disaster in Scottish history is commemorated. Overlooking the harbour is a striking bronze artwork depicting a long row of “widows and bairns” desperately scanning the horizon: each a representation of one of the hundreds of wives and children left bereft on October 14, 1881, when a devastating hurricane destroyed 19 of the 45 boats that had gone out that morning. Some 189 fishermen never came home.

In her book, Salt On Your Tongue, Charlotte Runcie includes a chapter on how to survive a maritime storm. “Keep the [ship's] bow pointing into the waves to avoid them smashing you from the sides,” she writes. “Breaking waves can capsize you.” If the worst happens and you find yourself clinging to the wreckage, “don't thrash around or try to swim”. Instead, stay calm, improvise a sea anchor from debris and if you are thirsty and can manage to catch a fish, suck out the fluid from its eyes and spine.

At 29, Runcie has never been to sea. So how, I wonder, did she learn all this? “I have a good friend who's a qualified sailor and I asked him how he prepares for a trip,” she explains. “When you go out in a boat, you are taking your life in your hands so you have to think about these things.”

Portobello is one of Scotland's most popular pleasure beaches and this morning, the waves lapping around our feet seem gentle and benign. Out there beyond the Firth of Forth, however, is the North Sea: one of the world's most treacherous oceans.

It was here that Runcie first conceived the idea of writing a book about women and the sea. At the time she was living in Edinburgh, working as a freelance journalist and pondering her future. She was in her mid-20s with a literature degree behind her but no idea what she wanted to do with the rest of her life. In many of the books she'd read, the passage to adulthood was marked by a nautical adventure. From Telemachus in Homer's Odyssey to David Balfour in Robert Louis Stevenson's Kidnapped, all the great literary heroes had confronted the terrors of the deep before making the transition from boy to man.

But what about girls, wondered Runcie, realising that for most of human history, life on the ocean wave has been a male preserve. Indeed, superstitious sailors considered it so unlucky to have a woman on board that during the Middle Ages, one storm-hit crew is said to have thrown all their female passengers overboard in an attempt to appease the sea gods.

Whatever the truth of that story, even within coastal communities, women's place has traditionally been on dry land, gutting and preparing the catch for sale while glancing anxiously towards the horizon, hoping that their menfolk would return.

From her own place on the shoreline, Runcie was spending her spare time wandering along Portobello beach or singing sea shanties in the community choir she'd set up at the local Dalriada pub. She was also sketching out the beginnings of a biography of the folk musician and sea shanty collector, Stan Hugill.

Two things conspired to blow her off-course. Firstly, the more she read of nautical folklore, the more intrigued she became about mariners' equivocal attitudes towards women. For despite all their superstitions, those men adorned their ships with female figureheads and referred to the vessels as “she”. And although classical adventurers of the high seas were all male, their maritime adversaries were often female.

In Greek legend there is Amphitrite, queen of the waves; Scylla, the many-headed monster who claimed the lives of men who sailed too close; and the Sirens, who lured crews onto the rocks with their beautiful singing. “Mermaids,” writes Runcie, “are known for tricking men at sea to drag them underwater and drown them.” And in the old English saga, Beowulf, the sub-sea-dwelling Grendel's mother is the most terrifying monster encountered by the hero, even if the Hollywood version portrays a hypersexualised version played by Angelina Jolie.

What these figures have in common, says Runcie, is that they are cleverer than the men they encounter: “Mythical sea women almost always have an understanding of the sea that transcends the knowledge of even the hardiest and most experienced male sailors.” That men have written stories about women's supernatural ability to control something they can't understand, “probably says a lot about male inadequacy fears”, adds Runcie.

The second thing that steered Runcie's book in a different direction was the discovery that she was pregnant. “That changed the way I thought about the sea,” she says. “Suddenly, I felt that I was at the mercy of nature. I knew that I was going to go through childbirth, which I was quite scared of. It was going to be painful and I wasn't going to have any control over how it happened. It made me realise I was less interested in tall ships and sea shanties than in the history of how people have confronted the idea of the overwhelming power of nature.”

Those people have often been women, who may not historically have battled oceanic tides but who have always been “brave and strong” in the face of overwhelming physiological forces. And the book Runcie eventually published in January, when her daughter was just over a year old, is a lyrical exploration of the complex relationship between women, the sea, and their own reproductive biology.

It's also part-memoir. For Runcie, bearing a child turns out to be the adventure she'd dreamed of and when the storm breaks. her description of childbirth – in all its visceral, growling, bloody intensity – is astonishingly powerful. How did she remember it so vividly?

“I wrote it straight after it happened,” she says. “I wanted to be really honest about it.” She feels we don't talk, or write, enough about this primordial experience – partly because it's dismissed as being commonplace. Yet until comparatively recently, childbirth was an extremely hazardous process. “It's still a really major thing for your body to go through, but in the past it might well have killed you.

“It's also something that makes you feel stronger,” she adds. “After giving birth I felt – what else could I do? You feel almost like a superhero, because you can't believe your body's just done that.”

If the shoreline is a boundary where lives change, it's also a place where lives end. Death is a recurrent theme in Salt On Your Tongue, partly because of the ocean's voracious appetite for human souls but also because Runcie had been profoundly affected by the loss of her grandmother, whose hand she'd held while she slipped away, her lungs struggling “as if she was trying to breathe underwater”.

Growing up in Hertfordshire, Runcie had an exceptionally strong bond with her grandmother – a respected concert pianist whom Charlotte recalls as “an amazing woman, quite forthright: very much her own person with her own interests at a time when it was quite unfashionable for wives of clergyman to be that”. Rosalind Runcie wasn't interested in church fetes or flower arranging and she always insisted on her husband helping out with the washing up – a story that gets even better when you know that her husband – Charlotte's late grandfather – was Robert Runcie, a high-profile cleric during the 1980s when, as the Archbishop of Canterbury, he officiated over the marriage of Prince Charles to Diana Spencer and famously clashed with Margaret Thatcher's government by criticising their policies on inner city poverty and praying for the dead on both sides of the Falklands conflict.

Runcie's parents are also well-known. Her mother, Marilyn Imrie, is a radio producer and her father, James Runcie, is a filmmaker, TV producer and novelist whose books about the sleuthing vicar, Sidney Chambers, have been adapted into the ITV drama series Grantchester. Clearly, the influence of Christianity has been strong in the Runcie family and in her book, Charlotte explores the role of faith in seafaring communities, whose church bells would be rung during storms to ask God to calm the waters. “Perhaps,” she writes, “they thought people's cries were hard to hear over howling winds, and bells are louder than prayers.”

Prayers and hymns did, of course, reverberate around Scotland's coasts and The Unst Boat Song – an ancient Shetland air – is a favourite of the Dalriada Bar community choir, with whom Runcie still sings. The song, she says, is “a prayer for safety at sea that probably would have been sung by women on shore, whose lives completely depended on their husbands and fathers who were away for months at a time, at the mercy of storms”. In many coastal communities, those prayers would have been made to Mary, who as Stella Maris or “star of the sea” was venerated as a protector of seafarers.

The constellations have been guiding sailors for many thousands of years and in her book, Runcie describes the Seven Sisters, or Pleiades, as “the sailing stars”. Alkyone, Elektre, Maia, Merope, Taygete, Kelaino, Asterope: say their names aloud and they sound like a spell, writes Runcie, who is an award-winning poet as well as a journalist and author.

Given her love for the sea, I'm surprised to learn that Runcie and her husband and daughter have made their home in a landlocked part of the Scottish Borders. Doesn't she long to live by the shore? Actually, she says, she finds it easier to write about the sea when she's not there. The ocean is so overwhelming, “so loud and big”, that when it's before you, it's hard to do anything except look at it.

I wonder if people have always felt drawn to the sea or if, in earlier, harsher times, they were simply too busy surviving to gaze in wonder at the distant horizon. “I think the literature and music that exists from those times tells us that they thought about it a lot and they told stories about it,” says Runcie. “All those folk tales that are hundreds and hundreds of years old, about selkies and other creatures … people must always have thought about it and been inspired by it.”

The sea is, she says, “a very beautiful place, a very romantic place, the place where we go on holiday beside but also, for thousands of years, it's been something people have been scared of. And we are still scared of it. I think every generation projects onto the sea a new kind of danger.”

She's referring to the distressing footage of desperate refugees setting out in the Mediterranean in overcrowded, unseaworthy boats, and to rising sea levels linked to climate change.

Then there is pollution. Salt On Your Tongue contains a lovely account of the origins of those little pieces of frosted glass that wash up on the shore, each a “souvenir of history” which “might have begun its life at sea as part of a gin bottle on an 18th-century ship … long wrecked and lying on the ocean floor”. It seems unlikely that the future of all that discarded plastic will be quite so romantic.

What does the future hold for Charlotte Runcie, who may not have gone to sea but has skilfully depicted its power within 350 beguiling pages? She is working on another book but will only say that it's fictional and “draws on the history of mythology and magic and the supernatural as well as the natural world”.

The tide is going out here on Portobello Beach and Runcie needs to get home to her daughter. Before leaving, she points out some of the islands in the Firth of Forth. There is Inchkeith and, just visible through the mist, the Bass Rock – home to the world's largest colony of gannets. Further out into the North Sea there may well be monsters, mermaids or perhaps even Sirens, waiting to lure sailors onto the rocks with their irresistibly beautiful songs.

If you close your eyes and listen to the wind, you can almost hear their voices, calling seductively to those in peril the sea.

Salt On Your Tongue: Women And The Sea by Charlotte Runcie is published by Canongate, £14.99. Charlotte Runcie will discuss the book at The Salt And The Sea, and Aye Write book festival event in the Mitchell Library, Glasgow on Thursday, March 14 at 6pm