Salted fish can also be found on
a rotting warchest (has like 60 ~ 80 hps)
located by the necromantic aide or his ph in Undershore.
The warchest will also sometimes give out various healing potions (I recall getting V and VII, there have also been others, though I don't remember specifically which ones) in place of the salted fish. I have not seen multiple drops on any single warchest; I've only gotten either a salted fish or a single healing potion.