Sobol, Donald J. My Name Is Amelia. 1994. Archive.org.

While sailing a sloop to St. Thomas, sixteen-year-old Lisa Maddock is knocked overboard and finds herself on an unknown island where she meets ten-year-old Amelia Earhart who has been snatched from the past as part of a fantastic experiment.

This was a story I was not expecting from the author of Secret Agents Four and creator of Encyclopedia Brown. Strange sci-fi story of a girl who falls overboard in the Atlantic ocean and finds a hidden island.