This one took me a while to read, not because it was bad but because I just fell off reading. So don't take the time it took me to read it as a bad sign. Cause everything else should pretty much clue you into that.
Leviathan is the third and, thankfully, last entry in the Cate Granger trilogy, a trilogy that shouldn't have even been made. Fathomless was a perfectly fine, if not very creative megalodon story where a bunch of people get eaten by a big shark and then the shark dies at the end. There was never really a need for a sequel but apparently Greig Beck just thought there was a little more plot that was necessary for the story to be complete.
My biggest problems with this book is that A, there was absolutely no need to continue the plot, it worked as a standalone and should've stayed that way. And B, that everything here just reads like someone trying to do Steve Alten's Meg books but with their own twists... except there are no twists to the plot here, it's just Steve Alten's Meg books with a new cover and a few new characters.
Fathomless read like a pretty straight forward copy of Meg. But so do most megalodon books because that's kind of the grandfather of the whole thing. Abyss read like a pretty straight forward summation of the first three Meg sequels but with a little bit more color added to it. And then there's Leviathan which just reads like Vostok but slightly better... which isn't much better.
I get that you're a fan of Steve Alten, Greig Beck, but why would you cover VOSTOK of all books? That was a terrible book (but also one of the most fun books to read the reviews of).
Everything about this book is just bad from the start. The two main characters are barely in this thing to start with and are only forced into the plot for... reasons. They're told they're the only ones who can do it and yet they never seem to contribute much specialty beyond saying the shark is right there and it's behaving like a shark. Which I'm pretty sure any old shark expert could do. Greig immediately establishes all this cool techno babble stuff that's supposed to make the shark into tiny little pieces but that would get in the way of anything interesting happening so that needs to go through some of the most contrived chain of events you'll ever read. Seriously, he starts laying the plot for how technology is taken out so early it feels like it would've been easier to just not introduce the technology in the first place. We've done the "human go in overconfident and nature punishes them" plot now to death, we definitely don't need more of it.
And to make sure the plot has absolutely nothing interesting about it, he introduces a bunch of Russians to bulk out the fodder count. And when he runs out of Russians to kill off, suddenly the protagonists are surrounded by named character freshly introduced so the shark (or whatever) can pick them off.
I think there was a grand total of one new character introduced in this book that felt like an actual character while all the others were there to be eaten or killed in some extravagant. Beck also has thing annoying tendency to cut away to entirely irrelevant deaths that add nothing to the plot and are there just for... I dunno, padding the page numbers or give script writers trailer shots for any potential movie he wants to be made.
The ending is also just bad, meant more to tie the books into the Alex Hunter series of books which... why? That universe is so complicated and convoluted with the various tie-in series now that I'm not sure what it adds? If I read a megalodon book, then I'm here for the megalodon, not the Kraken thing from Beneath the Dark Ice, no matter how much I liked that book.
And to end the trilogy on such a shitty book? Dear lord. What were you thinking, man?
I know I'm a broken record with this, but Beck and many other authors have this really annoying tendency to anthropomorphize their animals way too much. A massive shark like this megalodon going out of its way to eat humans is just weird and the book can never really decide whether the shark is out for vengeance, is just a mindless killing machine that obeys any and all instincts or is just plotting and scheming to get at the little bloody morsels that it eats like popcorn.
I'll sign off this review by saying I think Greig Beck's books have just gotten... worse in terms of writing. The sentences just don't flow like they used to, there's way too many grammatical and syntax errors, and the continuity is ALL over the place.
So, if you do like megalodon books and you absolutely need to read one written by Greig Beck, stick with Fathomless. It's the only one worthwhile to read in this trilogy.