The Mask of Love
Cartland, Barbara
The Marquis of Melford was actually relieved to be alone on his yacht.

Carnival in Venice was every bit as gay and abandoned as the Marquis had remembered, but after a few days the revelling had begun to bore him. Even the seductive charms of his dark-eyed mistress had worn a bit thin. Now the Marquis sat at the writing desk in his luxurious cabin, happy to be heading back to England, delighted his demanding mistress had decided to remain in Venice. Women were such a nuisance aboard ship, always....

The constant faint thumping noise behind him had finally succeeded in disturbing the Marquis' thoughts. It seemed to be coming from the large painted wardrobe that stood against one wall of the cabin. Opening the cupboard doors, the Marquis lost both his composure and his well-earned solitude. There, huddled in a corner, was the most beautiful young woman the Marquis had ever seen--a fragile, blue-eyed, golden-haired stowaway.

Caterina runs away from a loveless and frightening marriage and takes refuge on a Marquis' yacht en route to England. When the yacht is blown off course and encounters Barbary pirates, it is only then that she and the Marquis realise they have fallen in love.