Water Monsters (also known by their official name, "Kaernk's"), are monsters that PewDiePie frequently faces. They are, as their name implies, monsters residing in the flooded areas of Castle Brennenburg. The Kaernk's real form is unknown. The only way to detect if it is around is by the splashes it makes in the water as it "walks" and chases PewDie.
Whenever PewDie is in a area full of water he worries about the water monsters, and when he does encounter one of them, he tries to stay on top of floating objects or just simply runs around screaming like a girly man - not really paying attention to which way he is going & doesn't know where he is going too- and sometimes even stops to turn around and check how far the monster is (and occasionally takes the chance to throw boxes or other things at it), which usually ends up with him being devoured by it.
The water monster is one of the many characters that PewDie hasn't named yet.
Note: The Water Monster seems to eat any type of flesh as it eats a Bro's torso, arm & the like when PewDiePie throws one at it/in the water.