I think I have read somewhere that crocodiles may grow to be far larger than alligators,
and so more easily able to kill a swimmer. I have also read that they roll over very
powerfully, so that prey gets battered against the bottom of the water.

From the Internet:
The most believable record-holder for the longest
saltwater crocodile was a massive adult male killed
on the Fly River in Papua New Guinea in 1982. The
animal had already been skinned when a visiting
zoologist measured it at 20.3 feet (6.2 meters), and
because we know skins shrink slightly it's likely the
animal was nearly 21 feet long. It would have weighed
between 1 and 1.5 tons!

Here's figure for alligator. Not as much difference as I thought.

5. What's the biggest alligator in the Park?
No one really knows for sure. There are some that measure 15 to 16 feet in length that live here. (