Approximately 1.6 million tons of old ammunition are lying on the bottom of the North Sea and Baltic Sea, posing a considerable danger: their casings are slowly rusting and emitting toxic substances such as TNT compounds.
Most of the ammunition was deliberately sunk in the ocean after the war because the Allies were concerned that Germans would resume hostilities against them again at some point, and ordered that Germany destroy all ordnance. At the time the easiest way to do so seemed to be to simply dump everything into the sea.
Old science fiction books are exactly like this. They just assumed we’d have technological solutions to everything.
Also, they weren’t living in a largely collapsed ecosystem. Today we view this story in horror, but back then there were 1/4th the people, wildlife and nature was bountiful. It was probably hard to imagine that we humans could substantially alter the world. Hell, people today look into the sky and say global warming is bunk. Yeah, looks huge from down here! Take a look from space, paint on a marble.