My favourite is the sunken ship packed with explosives in the Thames estuary. As it rusts, it could explode at any time, sending a small tidal wave up the river and into London.

http://en.wikipedia.org/wiki/SS_Richard_Montgomery

Some very dark part of me wants to dive this ship.

Seems to be the way we dump that stuff. Waters of Italy house a time-bomb as well. http://www.euronews.com/2014/02/21/troubled-waters-the-hidde...

Also: Belgium waters have been a dumping ground for (mustard?)gas.