While I can't absolutely prove anything about your grandfather's story without more information, our Guide on Military Identification may be of use to you. It provides a number of different resources, including how to request service records from a number of national agencies around the world. This includes the German Navy, which would therefore allow you to fully explore your grandfather's story.

Your grandfather's story has four key elements to it that would allow us to narrow down which submarine he served aboard. Firstly, his submarine was either based in Norway, or was sunk close to it. If his sub was based there, it doesn't rule much out, as most boats were based there following the liberation of France. However, very few boats were sunk near the Norwegian coast. Secondly, the submarine was far from land or other ships, as he had to spend a considerable time in the water before being rescued. This rules out most sinkings by warships (in case lines were crossed somewhere) as the Allies were assiduous about rescuing U-boat survivors, as well as any sinkings in bases or close to land. Thirdly, narrowing it down to 1945 helps significantly; there were fewer boats sunk that year than in 1943 or 1944. Finally, the fact that he survived is somewhat significant, as many boats sank with all hands.

Very few German submarines were sunk close to the Norwegian coast in 1945. Of these, only one, U-711, was sunk by aircraft. However, she was sunk by a British air raid on the submarine base at Kibotn. This rules her out as the one your grandfather sailed aboard. Here, her survivors would have been easily rescued by small craft from the base. The survivors would also not have been taken prisoner by the British, but have been rescued by the Germans. As such, we must look further afield. Several U-boats were sunk by aircraft in the Kattegat, the strait between Denmark and Sweden/Norway in April-May 1945. This would be a stretch to describe as the coast of Norway, but still seems reasonable to investigate. However, most of the survivors from these boats would likely have been picked up by the Germans, not the British. Denmark was still under German control at the time when these boats sank. The one exception is U-579, which was sunk the day after Denmark was liberated by the Allies. She was depth-charged by a B-24 Liberator bomber of 547 Squadron RAF, near Aarhus. However, it is unclear whether or not there were any survivors from those aboard and she may have sunk with all hands.

Boats based in Norway made the majority of the patrols around the British Isles in 1945, and several of these were sunk by aircraft. The vast majority of these were sunk with all hands, but there was one with survivors. U-681 set off from Kristiansand in Norway on the 16th February 1945, for her maiden patrol in the English Channel. On the 6th March, her commander fired a torpedo at an anti-submarine trawler, but the weapon missed. This set the tone for the rest of the patrol. On the 11th March, entering the Channel, the boat ran into an underwater rock near Land's End. This did sufficient damage to force the boat to surface. On the surface, the boat was attacked by a US Navy B-24, of VB103. The aircraft was flown by Lieutenant Norman Field, who dropped eight depth charges around the submarine. The crew swiftly abandoned ship, jumping into the water. They were rescued from the sea by the British Support Group 2, with the frigate Loch Fada picking up 38 survivors. Eleven men died aboard U-681. This seems to line up with some of your grandfather's stories - the boat was ultimately sunk by an aircraft, the surviving crew were in the water for some time, and they were rescued by the British. Another possibility is U-1206. She was another new boat, leaving Kristiansand for her first patrol on the 7th April 1945 heading for the east coast of Scotland. She suffered engine trouble early in the patrol, meaning that she had to pass up at least one promising target. On the 14th, though, there was another mechanical failure. The boat had been fitted with a new high-pressure toilet, which could be used while the boat was at great depth. This failed somehow - one account suggests that the wrong valve was opened by an inexperienced operator - causing water to flood into the boat. Faced with a boat that was flooding and with chlorine gas building up inside, the boat's captain, Karl-Adolf Schlitte, chose to surface and abandon ship. While surfacing, she may have been attacked by an aircraft, though my sources differ. There were 46 survivors from U-1206. However, when she went down, she was close to the Scottish port of Peterhead. Most of the survivors were picked up by local small craft, but ten men did struggle ashore in a rubber raft, landing closer to Aberdeen. This is a more speculative connection to your grandfather's story; the presence of an aircraft is uncertain, and the submarine was so close to land that any rescue would be relatively quick.