A British hardcover came out in 1968 with the title The Dolphin Rider. There are 2 sequels - I found them enjoyable (if light reading.) Cover art is not credited, but I think a signature is visible in middle left of cover, 1" up from bottom of book. I'm pretty sure the same artist did the cover for Daughters of the Dolphin - also uncredited, but I think it might be Robert Emil Schulz, or less likely Bob Abbett.