I found this book entertaining, but I pretty quickly figured out what was going on when I realized that the main mystery plot essentially follows the one in the 1972 Scooby-Doo episode "The Loch Ness Mess". I'll try to avoid specifics to avoid spoilers. If you've seen "The Loch Ness Mess" you'll guess what's happening too even without me pointing things out.

It's not just that the good guy investigators in both cases arguably consist of a dog, two women, and two men. It's not just that the protagonist in the book shares the name Morgan with the main bad guy in the Scooby-Doo story. It's not just that both also have out-of-place Loch Ness ties in the U.S. It's that the details of what's actually happening and the motive of why it's happening are essentially identical in the two. Both stories feature monsters with yellowish eyes attacking small boats. In the Scooby-Doo version, it's just to scare people, while in the book people who don't get scared get murdered. One has gold connected to the Civil War while the other has gold connected to the Revolutionary War. The way that gold is used in the two stories and the odd way in which it connects to the monsters is pretty much the same in both.

There are some significant differences with subplots. The book has romance and tragic backstories. The Scooby-Doo episode has the Globetrotters and a few extra ghosts on land. The twist ending in the Scooby-Doo story isn't followed by the book (although there's one point in the middle where it almost hints that it's going to be) and the identity of the villains isn't identical in the two (although there's also one point where it looks like they may be).

So while it's a fun read I'm giving it a lower review than I'd otherwise give it. I don't think the author deliberately copied the Scooby-Doo episode, but I wouldn't be surprised if she'd seen it as a kid and still had a few details kicking around in her head when she wrote this book, even if she consciously forgot watching it.