The UB-85 sea monster claim

On 30 April 1918, the captain of the German submarine UB-85 reportedly told British captors that a sea beast had damaged his boat. Other accounts say it flooded during a dive.

Wikipedia records that Krech, commander of the UB-85, gave the first account of a monster on 30 April 1918, in an interrogation by British forces after his U-boat was captured. The account reached print sixty-nine years later, in James B. Sweeney's 1977 book Sea Monsters: A Collection of Eyewitness Accounts, which quotes Krech.

The Guardian retells the story this way: Krech explained to the men holding him that day, 30 April 1918, how the boat was on the surface because damage from "a strange beast" had stopped it diving. The creature had horns, tiny sunken eyes and glinting teeth, and it was so heavy that the whole vessel tilted sideways when it climbed aboard. It chewed lumps out of the forward gun. The crew surrendered to the British patrol, and the submarine was scuttled. Forces News says Krech reported that a large creature attacked while the boat sat on the surface by night, recharging its batteries.

Other records describe the same day differently. uboat.net lists 30 April 1918 as the day water entered through a partly open hatch as the boat dived away from gunfire by the Royal Navy drifter Coreopsis. It surfaced, and the crew left it while patrol vessels fired on them. All 34 crew survived. A BBC report quotes an account in which the submarine flooded from above once underwater, so the crew blew their compressed air, surfaced and surrendered.