The ones around Britain and the North sea look close to shore but mostly seem to be tens of miles out at depths of 1600ft or more (the ones between Denmark and Norway are 2100ft+) so the chances of people kids swimming near them or even pulling them up in fishing nets are pretty much zero.

So if its 1946 and you have to dispose of hundreds of thousands of tonnes of chemical weapons sitting around taking them out further seems like a waste of time and resources.