345U: Virgin Islands young adult mystery (Solved!)

I am searching for a young adult mystery that was set in the Virgin Islands.  The protagonist, a young woman, receives a message from her father on an alphabet shell.  I think it was her father, who is a doctor, and who has been abandoned on an island.
Please help me.  This book was a seminal influence on my desire to live on islands. I now live in Manhattan.

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