Number 17 in Randy Wayne White's "Doc Ford" series. If you look over at Amazon, you'll see that a number of readers were unimpressed, but I liked it quite a bit, sue me. The premise is ludicrous, but if I stopped reading books with ludicrous premises, I would eliminate a lot of items in my TBR pile. Which might be good, but I'd also have a lot less fun.
The story is that one of Doc's Dinkins Bay acquaintences, cantankerous Arlis Futch, thinks he has a lead on Cuban dictator Fulgencio Batista's lost "treasure plane", which was rumored to contain vast riches looted from the Cuban treasury, and vanished on its way off the island in 1959: could it have crashed in a small inland Florida lake? Futch thinks it's likely enough to purchase the lake and the surrounding land. He enlists SCUBA-skilled Doc to go exploring, and they bring along Doc's hippie friend Tomlinson, and the Native American/juvenile delinquent introduced in the previous book, Will Chaser.
Multiple disasters strike: the lake is unexpectedly delicate, and a catastrophic underwater avalanche traps Tomlinson and Will with their dwindling air supply.
In addition, a couple of ex-cons, fresh off a robbery/murder/rape spree, just happen to encounter the lake at the same time. They are desperate and violent, but also greedy. So Doc has serious problems above-water as well.
Could it get worse? Yes it could: the area is legendary for harboring a great, mysterious beast. Much worse than your average Florida snakes, gators, and crocs. That legend turns out to true enough to menace both the good guys and bad.
It's too long, a very common malady among contractually-obligated popular novelists. But you'll learn a lot about Florida lake geology, so that's a plus. I'm very fond of the Will Chaser character, and I hope to see him again. Final quibble: I would have liked to see a map, because I got kind of lost relying solely on text descriptions of the complex geography.