I don't remember the cover, but I remember bits of the plot. A mysterious man is investigating some creature in the lake that kills people. He's investigating it with a new woman who came into town, whom he got close to and had sex with in a cave. I think he was a twin too. Also, there was this old lady protecting the creature in the lake.
Hi, welcome to SF&F. When did you read this? Was it about Loch Ness?– DavidWApr 5 at 2:00
Do you have any information as to when it was published/read or any information about the actual book itself (ebook, paperback, hardcover, short story, etc)?– Blue Skin and Glowing Red EyesApr 5 at 2:00
This is a really long shot, but possibly Richard Laymon's "Dark Mountain".
Rough synopsis: there's a mysterious lake inhabited by a creature that kills people. There are some passing campers, two of whom are a new couple who have sex, but I can't remember if they do so in a cave.
Turns out the lake creature is really a murderous hillbilly being protected by his mother, an old lady who's really a witch.
Can't remember anything about twins.
This is also a bit of a long shot, but could it be Under the Lake by Stuart Woods.
In the beautiful mountains of North Georgia lies a lake built by an obsessed man at a terrible price. This placid body of water has brought prosperity to an isolated community, and with it, two strangers who intermingle with the insular local folk, strangers probing into crimes against nature from generations past that cannot remain submerged beneath the waters' surface.
The man investigating the creature in the lake is John Howell, an investigative reporter whose career is on the slide. The woman is Scotty MacDonald, though she is using a pseudonym Miller. Scotty is also a reporter and she is in town to investigate the local sheriff for corruption. John and Scotty end up having sex though not in a cave.
The woman protecting the creature is Ma Kelly. She is the matriarch of a local family deeply involved in the mystery.
The creature turns out to be:
Kathleen O’Coineen, who was murdered by the sheriff Bo Scully. Scotty turns out to be Kathleen's illegitimate daughter.