Stay, Sweet Love — Aileen Fisher

Summer boys almost never wrote to summer sweethearts after their ferries had carried them back to the mainland. Island girls often waited in vain through long, lonely months for letters that never came.
But that would never happen to Ellen, for she had Rob - her sweet first love. He told her that nothing could take her away from him ... not now ... not ever. She knew he would return to her as surely as the tides washed in and the moon rose.