Yes, unexploded naval ordnance has been a problem, especially in the North Sea and Baltic Sea. Mines were by far the most important contributor to this problem.

During both World Wars, many navies carried out large-scale mining operations, laying thousands of mines to create barrages and barriers. In World War One, the Royal Navy laid over 5,000 mines as part of a barrage in the Dover Straits. Combined with surface patrols and nets, they were intended to prevent any U-boats passing through the area. Following the USA's entry to the war, work was begun, at the insistence of the USN, on a similar barrage across the northern entry to the North Sea. Called the Northern Barrage, it would ultimately contain some 73,000 mines, laid at various depths between the Orkneys and Norway. The Russian Navy used 2,200 mines in the Baltic, preventing the German Navy approaching St Petersburg or raiding the Baltic coast of Russia, while the Germans laid ~250 mines in the entryways to the Baltic. Similar work continued in WWII. The British laid a massive mine barrage off the East Coast of Britain, protecting the coastal convoy routes. Three thousand mines were laid in the Dover Straits, and more were laid in fields off Scotland's North Coast, or in the St. George's Channel. The Italians laid over 17,000 mines in the Mediterranean, mostly along the routes to Malta. The Soviets conducted massive minelaying operations in the Baltic, laying 8,000 mines in the second half of 1941 alone. WWII also saw the advent of offensive minelaying by aircraft, with mines being laid in the approaches to ports or along shipping routes. British aircraft laid 48,148 mines, mainly in the Heligoland Bight. The American and British air forces also carried out a vast minelaying operation against Japanese targets, laying 4,580 mines in the Indian Ocean area, 1,239 near Chinese ports, 2,522 in the South-west Pacific and over 12,000 around the Japanese home islands. Over the course of the war, between 600,000 and 1,000,000 mines were laid. Sweeping all of these mines posed a major challenge in the post-war periods.

Mines were typically designed with a number of safety features that would, in theory, disable them after a certain period of time. These could include water-soluble plugs that would cause the mine to sink after a certain period of time, or timed charges that would blow the mine's case open. These simplified the job of disposal, but frequently failed. While mines were usually laid in known, marked fields, they often broke their moorings, and began to drift, complicating efforts to sweep the fields. With the sheer number of mines laid, it's not surprising that there have been occasional cases where ships have struck or found mines, even in relatively recent times. Mineclearing efforts were generally led by the minesweeping infrastructure built up by the belligerents. This was slow, dangerous work. Sweeping the Northern Barrage after WWI cost 23 ships, with several more civilian casualties in addition. After WWII, minesweeping efforts were slow, but generally successful. From June 1945 to January 1948, the Royal Navy formed the German Mine Sweeping Administration, consisting of 300 ex-Kriegsmarine vessels manned by German sailors, to clear the North and Baltic Seas. The USN took most of the responsibility for minesweeping around Japan, but would ultimately hand this off to the Japanese Maritime Self Defense Force. Even so, despite diligent minesweeping, mines still claimed victims. In 1949, a Belgian steamer detonated a mine off the port of Dunkirk, in an apparently clear (and frequently used) area. Relic mines, especially in the Baltic, are still a problem today (it's estimated that up to 50,000 mines still remain in the area), with NATO carrying out yearly minesweeping exercises in the area, both training minesweeper crews and reducing mine numbers for public safety.