According to Gracenote, "Ferryman's Daughter" is on Donovan's album "The Best of Donovan." However, I've been unable to confirm the identity of this album; it seems there were several albums released with the title "Best of" or some variant.

According to This site says the song was on Donovan's "Live In Japan: Spring Tour 1973," an album released only in Japan, and This page lists it on a couple of other "Donovan Bootlegs and Rarities."

This page lists an old song called "Riddle-Come-A-Dinky-Doo (The Ferryman's Daughter)"

I also found references to a legend about a convent on London's south bank, an engraving, and an opera called "Ferryman's Daughter."

But no lyrics.