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.

338Y: Infant alone on an island

An infant survives a shipwreck and ends up on an island where he lives alone for a few years. Not certain what age he is when some people (grandfather maybe) come searching for him, but not being accustomed to people, he hides from them. The book makes a point of mentioning that the boy’s hair is very blond to almost white. I remember this was a very tiny book size-wise; smaller than the average book at the time.  It was a library book from the young adult/children’s section and I read it in the late 60’s early 70’s.

329W: Caribbean Island Adventures With Photos

I am looking for a book that I think my parents must have brought back from a Caribbean vacation in the 70s. It was the story of a bunch of island kids, boys and girls of different ages, maybe set in the Bahamas, or maybe not that specific, but definitely in the English-speaking Caribbean. Any adults were marginal background characters. There was a hand-drawn map of their island that showed beaches, caves, meeting places, shortcuts, etc. It was essentially a picture book, but scenes from the story were illustrated with black-and-white photographs of black children (actors? the author’s kids and neighbors?) and island scenes. The combination of an exciting adventure story with actual photography was wonderful.

243I: Isle of Capri (Solved)

Children’s fantasy book from the 50s, 60s or 70s, something about “the Isle of Capri”. I think the cover was basic – black and white, or dark green and white. Perhaps there was a butler and a big house…there was some sort of magic going on, I think. The Isle of Capri might have been spelled “Capris.”  I remember that a child goes to a big house, a mansion, to live and there are mysteries and strange things going on. That’s all I can remember!