During World War I, the most hazardous place to be, relatively speaking, was not on the battlefield, but inside a German U-boat.
Throughout the war, Germany deployed 375 Unterseebooten, i.e. U-boats or submarines; 202 were lost in action, or about fifty-four percent. Similarly, of the 17,000 sailors who served in them, about 5,100 were lost due to Allied attack, mechanical failure, and accidents. That accounts for a fatality rate of about thirty percent. Compared to the number of combat-related fatalities among German land forces, around 2,000,000 or fifteen percent, those suffered on U-boats were proportionally higher.
By the beginning of 1917, Germany’s economy had become beleaguered by blockaded ports and its war effort threatened by American munitions shipments. In response, it resumed its policy of unrestricted submarine warfare on Allied merchant ships on February 1, 1917.
The renewed German practice proved successful, accounting for just over 3,700 vessels sunk, damaged, disabled, or captured in 1917 alone, and included ships of all types and sizes, from trawlers to military transports to luxury liners. Generally unable to dive more than fifty meters or remain submerged for more than two hours, U-boats sunk enemy ships by firing surface torpedoes or laying mines.
As the map above illustrates, 1917 was a productive year for U-boat attacks on the Mediterranean Sea, with its comparatively mild weather and numerous choke points restricting passage. Each symbol of a sunken ship represents a vessel, regardless of tonnage, sunk by a U-boat after twelve months of unrestricted warfare, probably from February 1917 through February 1918. A note indicates that ships destroyed by mines prior to February 1, 1917, are not included, while a quotation from the November 4, 1917, edition of the New York Times testifies to the U-boats’ effectiveness by laying Italy’s defeat on its lack of ammunition and heavy guns brought about by the disruption to its ore and coal supplies.
Sailing out of an Austro-Hungarian naval base, such as the one at Kotor in modern-day Montenegro, the twenty-three-or-so U-boats assigned to the Mediterranean terrorized the region and the Luso-Hispanic coast. Who were the likely victims of unrestricted submarine warfare in those areas? Among them could have been Greek and Italian fishing boats, a Portuguese anchovy trawler, a Spanish yacht, a North African dhow, an American munitions transport, a French cruiser, or a British hospital ship.
In other words, the Germans were indiscriminate in their use of unrestricted warfare, which ultimately backfired. Just hours after the United States broke diplomatic relations with Germany over the issue, the American converted liner S.S. Housatonic was sunk by a U-boat in British waters. The sinking of four additional U.S. merchant ships led the United States to declare war on Germany on April 6, 1918.
The Allies finally overcame the loss of shipping by employing escorted convoys, and by effectively using depth charges and heavily armed vessels decoyed as merchant ships.
One Internet-based resource with much helpful information on WWI U-boats, including a list of all U-boats commissioned before the end of the War, is uboat.net https://uboat.net/wwi/.