Striking images of adventurous day-trippers scrambling onboard wrecked first world war German submarines off the coast of Cornwall have been released to mark the centenary of the day life at sea became infinitely more perilous for British mariners: 1 February 1917, the day Germany declared unrestricted submarine warfare, meaning that its U-boats would torpedo any British ships or those trading with the UK without warning.

Historic England is also upgrading to Grade II* the national memorial to submariners on the Thames embankment at Temple Pier in London. The sculpture by Frederick Hitch, showing a cross-section of a submarine surrounded by allegorical figures, was unveiled in 1922 in memory of the third of the submarine service’s total personnel who died in the war, the highest percentage of any of the armed forces. Submarines had been moored near the pier as early as 1907 and in August 1916 a captured German U-boat was moored there as a tourist attraction.

Facebook Twitter Pinterest Naval visitors on a listing UB-112. Photograph: Historic England

Roger Bowdler, director of listing at Historic England, said: “The declaration of unrestricted submarine warfare in February 1917 was a decisive moment in the first world war. Germany’s tactic led to devastating losses for many nations, but it also horrified the world. It was seen as uncivilised, ungentlemanly, and ultimately brought the might of the United States into the war.”

According to Historic England, 431 British ships were sunk by U-boats in 1916, but in 1917 the losses almost tripled to 1,263. Convoys with merchant shipping escorted by warships across routes including the North Sea and the Atlantic were adopted as a defence against the U-boats.

The photographs of the stranded German submarines were taken in 1921 by Jack Casement – a distant cousin of the human rights campaigner Roger Casement – who had been decorated for his wartime naval service and went on to work in the submarine research centre in Falmouth.

Facebook Twitter Pinterest Captain Jack Casement photographed the wrecked submarines during what was probably one of his last official postings. Photograph: Historic England

Serena Kant of the Historic England listings service has been researching the postwar fate of surrendered U-boats, which were stripped of anything useful including engine components and lighting fixtures, and then towed to sites around the UK coast. Some that were opened to the paying public to raise funds for charities became popular attractions, and were recorded in commercial postcards. Others were sunk as underwater gunnery targets, but several – including those Casement photographed at Falmouth – broke their tow lines and were driven ashore on to rocks.

Kant was contacted by Casement’s family when she published an article in a conservation bulletin – with an image of the submarine which broke its tow and ended up stranded on Hastings beach, where it became a much photographed local landmark until it was broken up for scrap.

Casement’s descendants donated 30 of the photographs he took around the time of his retirement in 1921 to the Historic England archives. Research continues on the six submarines which ended up at Falmouth, which had all been formally surrendered at Harwich in November 1918. Parts of some of the wrecks have recently been identified on the seabed.