Showing Heart's Passage by Cate Swannell

Jo Madison is a yacht skipper in the Whitsunday Islands of Australia's Great Barrier Reef. She has a dark and violent past she is trying to leave far behind her, and the last thing on her mind is love. But when Cadie Jones, long-time partner of a US senator, sails in to her life, her priorities change.

There's no rest for the formerly wicked, however and Jo's past comes back to haunt her, throwing herself and Cadie into mortal danger. Jo is forced to rely on skills and weapons she had thought long-buried, while Cadie struggles to balance her Midwest life against her attraction to the mysterious woman.

Set in the splendor of Australia's tropics, Heart's Passage traces Jo and Cadie's rocky path to an uncertain future.
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This is the first book of two - the sequel is called NO OCEAN DEEP.