I hate it when the tagline on a movie has the catch all phrase, "Inspired by True Events". Hell, INDEPENDENCE DAY is inspired by true events, HARRY POTTER is inspired by true events. It can apply to anything, because anything fictional is "inspired" by something that's true. Okay, got that out of the way.
The Australian movie BLACK WATER is a realistic seeming account of three people who get stranded out in the mangrove swamps in Northern Australia and held captive by a large saltwater crocodile. It won't let them swim to their boat, which is a short distance from the tree they are perched in. One of the characters tries to retrieve the craft, but is dragged under the water and killed and another is nearly killed when she's dragged to one of the creature's food hideaways a short distance away. The movie reminded me a lot of OPEN WATER, except it's on a river and it's a crocodile instead of sharks.
The croc itself isn't a giant, mutant creature, but a normal sized crocodile, about a dozen feet long. But that's big enough to be a big threat. BLACK WATER is one of the more realistic killer animal movies I've seen.