I know that there are many active landmines leftover from armed conflicts that harm civilians in many places around the world but I really haven't heard much about naval mines still floating around the seas.
They're not really floating around, but fishermen on the North Sea do catch mines, as well as bombs, in their nets, they also wash ashore occasionally. Why are there bombs in the North Sea? You may ask. Well, bombers were not allowed to land with their payload, so, if they could not find a target, or were forced to go back early, they would drop their load in the sea.
In 2005 the Dutch and Belgian navies, together, started an operation to clear the North Sea of explosives. Since then, they've disarmed about 1100 source
EDIT: to be clear, the majority are from WWII
Yup, there are still many active projectiles at sea and under France beaches.
Only a month ago, a man was killed and two of his friends gravely wounded by a 60mm artillery shell that exploded under their fireplace while camping on a beach, at Groix (in France). Source in French
One of the commanding officers charged of this investigation, Lionel Delort, says that an estimated 10% of explosives and mines used during WW2 didn't explode but the problem is that our own army dumped a lot of explosives at sea between 1914 and 1970 because it was just the cheapest way to get rid of them.
So at sea you have fishermen catching explosives and mines every now and then ... and inland, due to natural erosion, more and more explosives are coming back to the surface and have to be disarmed (we pretty much have bi-monthly demining operations in Normandy these days)
Yes, there are about 40 000 mines still in the Baltic Sea, from the roughly 165 000 laid during ww1 and ww2. Most are inactive and sunk by their anchor, but every year there are some that shed their anchor and drift to land where explosives experts take care of them - this usually happes during the thaw, when the ice breaks up and at times bring mines with them.
During the Nord Stream gas pipeline project, the consctructors had to clerar a corridor of 50 meters for the pipeline and removed many mines.
It is still dangerous to sail outside the approved sea lanes with steel-hilled ships with a certain draft in the Baltic Sea.
Here's an image from the Swedish Coast Guard showing dangerous zones in the Baltic and North Sea.
We just had one found a few weeks ago off of South Australia: http://www.abc.net.au/news/2014-09-08/exclusion-zone-set-up-around-underwater-mine-off-sa-coast/5727944
