This is the book, I believe. I've been searching for the answer too. I read it for a book report in middle school in the eighties.

Deadly Sleep is the book, I believe. I've been searching for the answer too. I read it for a book report in middle school in the eighties.

Jaynie visits her friend Evelyn in Scotland to take a break from her problems at home, including a painful break-up with her boyfriend. Just as she begins to relax, she starts hearing eerie voices in the night, and when she investigates, she sees a ghastly figure rising from the slime of the lake.

This is the first book of the Twilight: Where Darkness Begins series.