Dangerous Depths by Colleen Coble
Leia ditched a promising medical career to settle on a secluded island of Hawaii. She ditched Bane, too, and he's come to the island to find out why. He's also in search of a fortune rumored to lie offshore. But an act of sabotage that pushes Bane closer to Leia plunges them both into a tangle of emotion--just as a series of threatening events grip the island.
Theft, a friend's death, a bizarre intruder, hints of a second treasure...and even murder--how can they sort it all out when everyone on the island has something to hide?
This is the last book in the Aloha Reef Series. This was another book that I had a hard time figuring out who the bad guy was before it was reveled. In my opinion it seemed to have left a lot unsaid. I wonder if she had thought about doing a follow up book on everyone in the series.