Although generally considered unaggressive, a kappa can be absolutely vicious when angered. While particularly famous for challenging unwary passersby to mano a mano wrestling matches, they are also notorious for ambushing and drowning those foolish enough to swim in isolated or fast moving waters. One strategy is simply to drag a victim below the surface. Another is to remove a swimmer's intestines from below, by punching a slimy hand through… well, use your imagination. The kappa isn't after the entrails themselves, but rather the shirikodama, a mysterious organ said to be located in the colon.
(From 'Yokai Attack! The Japanese Monster Survival Guide' by Hiroko Yoda and Matt Alt, 2008).