Piranhabane, an artifact rod that instakills piranhas (and possibly, turns them into tins of piranha meat in your inventory or at least out of the water).

When a corpse (possibly only a fleshy, blood-bearing corpse?) falls into water, any nearby sharks and/or piranhas become “frenzied”, gaining bonuses, for some time. Possibly they could also try to converge on the space containing the corpse and maybe eat it.