Dracoleech

The Dracoleech is a horrible Undead monster found almost exclusively in the River Grismerie between Mousillon and Bordeleaux, though some have been known to swim further upstream. The creatures are serpentine in form, between twenty and thirty feet long and about four feet across. Their skin is rubbery, slimy, and rotten, and there are no bones within. For mouths, they have circles of teeth, constantly pulsing, that can swallow a child or Halfling in one gulp.

Their preferred mode of attack is to come up under a boat, tipping any passengers and crew into the water where they can be chewed at leisure. They can also jump onto a small boat, smashing it, but they cannot stay out of the water for long.

Source

 * Warhammer Fantasy RPG 2nd ED -- Knights of the Grail (pg. 59).