Beneath Black Sails by Clare Sager

Sometimes it takes a pirate to catch a pirate.

With weather magic on her side, Lady Vice is the bane of the high seas, but she isn’t captain of her own ship. Yet. If she can persuade her captain to give her a command, she’ll be in charge of her own fate.
To pay off his family’s debts, Knigh Blackwood hunts pirates for the Royal Navy. And he’s damn good at it. When the bounty on Lady Vice increases, he’s determined to make her face justice, even if that means using unorthodox methods.
Forced to work together, neither can deny their mutual attraction. As they face battles at sea and schemes amongst their crew, they discover hints about a long-lost treasure that could be the answer to both their problems.
But treasure isn’t the only thing buried. Secrets best-forgotten lie in wait that could blast them apart. And the closer Vice and Knigh grow, the greater the threat – to her freedom and to his family. Because for one to succeed, the other must fail.