Its pretty simple really, it has nothing to do with magic pollution in the deep seas or anything like that. In fact im going to put a censor over it because your mind will be broken after you read it
The magic was taken by a fish
I have multiple points to prove this, as humans haven’t found it to this day, because it was taken by a fish, and you may ask “but how is that fish alive?”, and my answer to that is that it has the curse, therefor it cannot die.
“A Supercurse User will explode if more than half their body is submerged in seawater.” - AA Wiki
The Promethean Fire curse that was once carried by Morock is now basically submerged and so far under the water. Fish are basically fully-aquatic animals, so they’re almost always underwater. So even if the Fish could even absorb the curse, it’d be a death sentence.
The peacekeeper had to learn a very special technique in order to destroy the curses therefore, some piss wouldn’t have ended the Promethean Fire Curse.