EDIT: Very important edit in 2nd paragraph.

In World War II, the Axis and Allies laid a staggering number of mines; 600,000 in the North Atlantic alone. Most were deposited in well-charted minefields, but many were dropped by air and therefore drifted off-course. Others broke loose from their chains and began drifting.

Minesweeping was an important postwar cleanup task; much of the work was undertaken by the German Mine Sweeping Administration, a British-run organization that employed 27,000 German sailors. However, the number of mines made it inevitable that some would escape notice, even in the best-charted minefields. In addition, many mines were not hauled up and disarmed: while about 3,000 floating mines were simply cut from their anchors, thousands of contact mines were left to sit on the ocean floor. That made them mostly harmless, but larger vessels can strike them, dredging vessels and fishermen can accidentally drag them up to the surface, and storms can roll the mines into the path of boats or even onto shore.

Mines are still discovered on a regular basis around the world - sadly, often with deadly results. Hurricane Sandy dredged up several mines and brought them close to shore; here are photos of the U.S. Coast Guard detonating one in June 2013. Here's another one discovered and detonated in the Netherlands in February.

In 2009, NATO published the report "Sea Mine Countermeasures" - estimating at that time there were still over 80,000 live mines in the North and Baltic Seas alone.

This is far from the only danger left over from World War II. On land, Germany disposes of 200 tons of mines, bombs, and other unexploded munitions every year. In addition, there are dangers at sea besides mines; thousands of tons of German munitions were disposed of by dumping at sea after World War II, including huge stocks of chemical shells. Fishermen and tourists are injured on a regular basis after exposure to decayed high explosives and even mustard gas.

I can't provide the sort of comprehensive answer you're looking for, but you may be interested to know that in many coastal towns in the UK disarmed mines from both world wars are often used as collection boxes for maritime charities.

This one's at Sandown, Isle of Wight. https://farm8.staticflickr.com/7310/10944084766_0d73455b67.jpg

And here's one at Lyme Regis http://www.panoramio.com/photo/94374093

(Mods: if this it too trivial please accept my apologies and delete.)

Some still round certainly - a quick search of news online has this from yesterday. http://www.newsinenglish.no/2014/04/02/wwii-mines-exploded-in-oslofjord/

Norwegian and Swedish defence mine sweepers blew up five unexploded World War II-era mines outside Nesodden in the Oslofjord on Wednesday. It’s estimated more than 1,000 sea mines remain in the waterway.