The notion that lies can be a force for good rather than evil dominates Susan Fletcher's fourth novel.

Rather like Joanne Harris, Maggie O'Farrell and Anita Shreve, Fletcher is the kind of women's novelist who is classed as "high-end commercial": she is a writer of superior abilities who somehow lacks the capacity to really go for the jugular, to be quite tough enough, rigorous enough.

This latest novel will do nothing to change that classification, while at the same time providing plenty to enjoy. Smooth, flowing and feminine, Fletcher's prose lulls you with its lyricism and emotional resonance, but that flow depends upon a certain amount of repetition, a slight bloatedness that lets you skim pages without losing the thread of the narrative. It's a kind of superior romance, really, and while that rarely wins the biggest book prizes, it is a genre many are happy to see grow.

That's not to say Fletcher doesn't take risks – she risks making some characters unpopular or unappealing, as she does here, with the young woman Rona who is desperately trying to steal her troubled lover, Nathan, away from his artist wife, Kitty. She risks incredulity, almost a brush with whimsy, as she shows a tiny island community who believe the man washed up on their shore is the mythical Fishman who comes when called upon, when hope is almost gone.

The novel centres around Maggie Bundy, widow of Tom Bundy, who jumped into the waves to rescue a young man and lost his life saving him. Tom's mother Emmeline has been inconsolable ever since while his brother, Nathan, feels he should have been the one who died, and not Tom. When Maggie hears a man washed ashore has been taken to Tom's aunt Tabitha, the island nurse, she wonders for a moment if it could be Tom – and when she sees him, she is struck by his likeness to him. So struck in fact that, when she sees him in the garden, recovered and wearing some borrowed clothes of her dead husband's, she attacks him.

But that moment of hate turns to love as Maggie and the Fishman draw closer to one another. It is Tom's paternal aunt Abigail who keeps the books of myths with the drawing of the Fishman in its pages – a drawing showing a startling resemblance to the newcomer. And all those who meet him feel a sense of peace, even Maggie, even as she is attacking him. The island, as it transpires, has some secrets: the Bundy men, going back to Tom's father Jack and his father before him, are violent, short-tempered men, good with their fists and happy to take their temper out on both their wives and their children. This island, Fletcher makes clear, has been unhappy for a long time.

Such a grim past – Tom's aunt Thomasina drowned when she was a young girl, and Fletcher hints at incest and physical abuse – has made for a grim present where the islanders almost have to believe in myths, have to believe in the Fishman, come to give them hope. As Abigail's husband, blind Jim Coyle, says to the stranger, "Don't tell them the truth. Let them believe the lie – it is what they need." Some lies, some secrets, have been bad for these islanders, but now it is time for lies to save them.

The intrusion of a stranger into a small community has almost become a literary cliche, but Fletcher breathes new life into it. As well as depicting Maggie's stunned reactions to her feelings of love she explores Rona's passion for married Nathan, and Emmeline's refusal to leave the man who beat her for years. Occasionally it becomes hard to distinguish between the different women – the family tree at the beginning of the novel is a help – and the similarity of their voices is a consequence of the strength of Fletcher's. Her strength therefore is also the novel's weakness. It seems strangely apt for her fishy, mythical tale of love and loss and the depths that grief can reach.

The Silver Dark Sea

Susan Fletcher

Fourth Estate, £16.99