If Deepstar Six was a nautical riff on Alien, than Leviathan would be an aquatic version of John Carpenter's The Thing. A group of people in a remote and inhospitable environment stumbles across the wreckage of a foreign expedition and unwittingly returns with a shape shifting lifeform capable of infecting the entire crew. The details, some of them, may be different, but it's hard to ignore the often striking similarities. In both cases the lifeform in question spreads like a disease, infecting a host and gradually taking them over. In both cases if a piece is cut off of the lifeform, the creature can grow from both the original body and the new smaller piece. The only real difference often seems to be the locale.
One thing Leviathan lacks, though, is the often apocalyptic sense of paranoia and claustrophobia that The Things has. There's a couple of character's who keep their infection a secret from the rest of the crew, but there's never any real question about who may or may not be infected, and once they are they die in fairly quick order. Aside from a creepy scene involving the scavenging of a sunken Russian ship(watch for the oversize, mutated fish skeleton on the ocean floor which is completely unremarked upon by the characters), the movie goes for a more straightforward action/horror feel. This isn't too say Leviathan is a horrible film, in fact it's quite fun at times. This is aided primarily by Peter Weller's standard idiosyncratic performance and Stan Winston's standard gorgeous creature design(something also shared with The Thing; although Rob Bottin did the bulk of effects on that film, Stan Winston helped out on some of the more memorable effects). The monster looks great, a monster that keeps some of the physical characteristics of everything and everyone it kills, but never loses the look of a giant fish.
As I said, the film is often fun, but also often groaningly overdone. Take the ending, where the surviving characters escape from the ocean floor and run into their mysterious corporate liaison, seen up til now only on a video monitor, and played by the always slightly creepy Meg Foster. The movie, which has been trying to build up horror and claustrophobia, ends suddenly with the death of a main character and a stupid punchline. I guess that's a pretty standard way to end an action movie, but still a bit tonally off for a horror movie. All in all, a decent movie despite it's faults, but in the future I'll be sticking with The Thing.