For sheer longevity, there's the Hai Shi, ex. USS Cutlass (SS-478), which entered service in March 1945 and, last I heard, is still in service in the ROC Navy as a training boat.

U-505 was suffering not only from the weather, but from what, from a naval viewpoint, could be viewed as a considerable lack of maintenance. Ships have to be kept painted, and you have to be pretty aggressive about removing rust as soon as it appears. That wasn't being done to nearly the extent it should have been.

The relatively short design life of the WWII German boats may also have been a technological imposition. The design bureaus were thinking in terms of high speed, deep diving boats well before the war started, but they also had to think about what they could produce "right now." Most of what the Germans used represented an incremental advance over their WWI designs, but nothing revolutionary. That didn't come until it was too late to accomplish anything, and even their advanced boats turned out to have more than their share of unanticipated problems.