This book was about a kelpie (or sea demon) who disguised himself as young man.
It killed girls by kissing them - when kissed sea water would run down their throat drowning them. The bodies would be found nowhere near water with seaweed about. A specific girl was target. I thought the book name was something with Lavinia. The girl was a descendant of someone known to the kelpie and I think he thinks she is her. And this woman from the past was Lavinia. I really loved the book as a kid. Would love to find it again.