The Mermaid Summer by Mollie Hunter

About a hundred years ago, they say, a mermaid ruled the cold sea that washed around northern lands. She was beautiful and dangerous, and the fishermen of those parts were careful to speak well of her. All but Eric Anderson, who scorned her very existence–until the day the mermaid smashed his fishing boat against the rocks, nearly killing him and his crew. Eric bid his family good-bye, for after that, no one would sail with him. His grandchildren, Jon and Anna, never stopped missing him; so from the moment they saw the mermaid for themselves, they knew it was up to them to tame her and bring their grandfather home again…