I laughed so hard when I saw this movie, not because the plot itself was particularly funny (it is lighthearted and good for young kids, provided they're not frightened by funny looking frog monsters), but because, having lived in northeastern Ohio most of my life, it was obvious to me that none of it was really true to form. I could believe mutated frog creatures came from Lake Erie (couldn't you? It wasn't suitable for swimming for a while), but let me tell you, the scenery of Southern California is nothing like the scenery around Lake Erie. I could even see a hint of the mountains in the background! The area around Erie is so ridiculously flat, there's no way they could have actually been there. There is a rumor in reality that there is some sort of large sturgeon like creature in the lake, but witches? Not really our thing. Underwater caves? Possible, but I haven't heard of any extensive cave systems. Environmental mistakes aside, moonstones are not sapphire blue. It being a kids movie though, I can let most of that go, and just be amused at yet another reference to the places I grew up. I often wonder if the people who write these things have ever been.
As for the plot itself, it was fairly mysterious in Scooby Doo fashion, though I've found that most modern Scooby Doo productions have allowed a little bit of magic to seep into the story, and concentrate way too much on the relationships between the four. I was always under the impression that Fred and Daphne had a thing, though no one really talked about it, and other than that, romance was out of the equation, or had with special characters outside the group. But recently, people keep trying to push Shaggy and Velma together, and (as far as I'm aware) never before this movie and the most recent cartoon series Mystery Incorporated has there even been a hint of attraction between the two. It just seems so... misplaced. And in both productions, the two end up apart for one reason or another, which begs the question, why put them through the awkwardness in the first place?
This movie is great for kids as I said, but if you're a Scooby Doo purist, I'm not sure you'll enjoy it.