Corrected entry: A Fish and Wildlife employee, a country-bred sheriff, and a "great white hunter" - and they place a baited trap for a giant croc right next to their campsite.
Corrected entry: I find it incomprehensible that the characters weren't aware of the prior crocs. Fish and Wildlife and the County Sheriff's Department would be hard-pressed to write off the demise of numerous employees as "accidental drownings".
Correction: You may find it incomprehensible, but apparently the characters in the movie didn't. As such, at best it's a character mistake, but since it's based on your assumption, it's probably not even that.
Corrected entry: The first time the sheriff, Emily, and Frank go out on the lake, she spots something in the water. The sheriff motors to the vicinity, then shuts the motor off, drifts, and starts trying to fish the object out with a boathook. Emily strips down and dives into the water to retrieve the head. If the water was deep enough to dive into, why did the sheriff shut the motor down and drift?
Correction: Character decision, not a mistake - he wanted to lie still to fish the head out and avoid getting wet.
Correction: At least they won't have to go far to get the trapped croc, then, eh? Just because an action is not something you would have done, does not mean the characters are not allowed to choose to do it differently.