New object, a fishing rod. It can be applied to adjacent water squares to possibly catch piranhas or snag small items from the bottom of the water.

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.