Real math?
Okay, it weighs 18,000 lbs. The density of regular water (not salt) is 62.4 lb/ft3. So the volume of the beast to be able to float in regular water must be 288.4 cubic feet.
They said its mouth was 4 foot in diameter. So lets assume a simple cylindrical shape, 4 foot in diameter. The cross sectional area of this shape is 12.5 square feet (=PI/4*Diameter^2).
That means the beasty would only have to be 23 feet long to be able to float in fresh water.
More to the point? How many croc hunters could it eat in one bite.