Book about a girl that grew up in the Caribbean and learned to swim and deep dive. Later, moving to Europe (England?!) to escape from her past after the boat she grew up in, capsize and drowned her guardian.

She was married off to a wealthy and abusive powerful man. One stormy night, she hit him and ran off. She went into hiding on a boat with a childhood family friend that taught her how to deep dive. They traveled through the islands in Caribbean and one stormy day / night, their boat capsized and trapped her guardian and he drowned. Somehow, she ended up and lived in Europe with a family consisting of two brothers and a sister. Later on, they found out the dead husband is still alive and came to claim her and take her back. In actuality, he wanted her to show them the location of a lost treasure in the ocean that her guardian knew the location of. I think the brothers' and sister's last name was Lockhart or something similar.