Ships hunting subs faced a sort of odd challenge when it came to confirming their number of kills. After all, their target was often underwater, there weren't always a lot of other ships around to confirm the kill, and the destroyed target would sink additional hundreds of feet under the ocean.
"Are you sure you killed the enemy sub?" "Umm, I filled the ocean with explosives. Does that count?" "No, but that sounds awesome."
But sub hunters came up with a solution. See, most of a sub sinks when it's destroyed underwater, but some items float. These items include oil, clothes and the personal belongings of submariners, the occasional packet of documents, and, disturbingly enough, human remains.
It's definitely kind of nasty, but it's also good for ship commanders who need to prove they actually sank an enemy sub or five. Commanders would take samples of the water or collect pieces of oily debris.
In Britain, it was traditional by World War II to dip a bucket into the water, scoop up the soup of oil, seawater, and debris, and then keep it on the ship, often in the freezer or refrigerator if they had one.
"We took this photo as we dropped bombs on the sub. Good enough?" "I mean, the sub still looks super intact in this photo. Not good enough."
(U.S. Navy Reserve)
When they returned to port, intelligence officers would take the buckets to confirm the kills and collect what other info they could.
Obviously, a pile of documents or sub gear was preferred, but the bucket would do when necessary.
This physical evidence of the kill was important, and some ship and boat commanders failed to get credit for claimed kills because they brought no evidence.
"This time, we filled the ocean with explosives, and then took a photo of the second, larger explosion that followed." "Eh, guess that'll work."
There were other ways to get kills confirmed. If multiple ships had hydrophone and sonar operators who heard the sub suffer catastrophic danger before losing contact with the sub, their crews could confirm the kill. Or intercepted intelligence where enemy commanders discussed lost subs could be matched up with claimed kills. Photos were great for subs that were sunk near the surface.
But the preferred method was always physical evidence.
It became so well known, however, that some sub commanders would pack a torpedo tube with random debris and then shoot it into the ocean when under attack. The bubbles from air exiting the tube combined with the trash floating to the surface could fool attackers on the surface, giving the sub a chance to escape after the surface ship left.
The Japanese I-26 submarine, a legendary sub presumed sunk in October, 1944.
Eventually, this caused commanders on the surface to prefer the collection of human remains that floated to the surface. Since it was very rare for submarines to carry dead bodies, that was usually a safe proof.
All of this makes it sound like confirming submarine kills was an imprecise science — and that's because it was. After the war, governments exchanged documents and historians and navy officers tried to piece together which ships killed which other ships and when. Most ship crews saw an increase in their total kill count, since previously suspected kills could now be confirmed.
But some who had previously gotten credit for kills later found out that they were duped by decoy debris — or that they had gotten a confirmed kill for a sub that actually survived and limped home.