A Ring of Endless Light.

When 15-year-old Vicky Austin, her sister Suzy and little brother Rob visit their grandfather on Seven Bay Island, Vicky faces several unexpected challenges. Her beloved grandfather, retired Reverend Eaton, seems to be seriously ill, but tries to pretend that nothing is wrong. Vicky met the rich but emotionally troubled Zachary Gray the previous summer, and he reappears to renew the acquaintance. Another boy, 17-year-old Adam Eddington, recruits Vicky to help him with a research project, working with a dolphin called Basil. Vicky discovers she can communicate telepathically with the dolphin and his mate - and possibly with Adam as well