Mysterious Fathoms Below (Vow of Vengeance #1) by J.A. Good

Every culture has legends of monsters lurking beneath the waves. What if they were all real?

Rosamund Bailey was once hopelessly in love-until betrayal shattered her heart. A year later, she's trying to rebuild her life in London society. But when her ex-fiancé returns with a threat that could ruin everything, she's forced to make a dangerous choice.

Desperate, Rose hatches a reckless plan: find a long-lost treasure hidden beneath the sea. The only man who can help her? A notorious pirate captain with a past he can't outrun-and the same stranger she once shared a one-night stand with.

Now they're trapped together on a ship full of secrets, longing, and sea monsters.

Because the treasure she seeks isn't just buried-it's guarded.

Water wraiths. An ancient sea dragon. Kelpies that whisper your name.

And a pirate with eyes like the storm who might ruin her more than any curse.

If she's going to survive, Rose must decide which is more dangerous:

The monsters below the waves... or the one she's falling for. Perfect for fans of forced proximity, pirate x noblewoman, sea monster lore, and slow-burn romantasy with bite.