This has always been an interesting topic. The problem of course, is that most U-boats which went too deep never came back up...
I read through Blair's 2 volumes on the U-boat war, took me about 5 years off and on and only finished last year. He noted when U-Boats went to unusual depth, since this would have been noted on the captain's log, namely:
1. the deepest recorded dive occurred on march 11, 1943, U-432, a veteran VIIc on its 8th war patrol but w. a green skipper sank a merchantman. Incredibly, instead of taking evasive action, the boat stayed at periscope depth and the officers broke out champagne to celebrate...
. When the corvette
Aconit carried out a standard sweep, it was surprised to find U-432 still at PD.
Aconit dropped 10 DCs which caused extensive damage to U-432 and drove it down to
1,000 feet (307 meters). This depth "...so terrified all hands.." that the skipper decided to surface at which point it was quickly sunk by the escort, so we have no idea what damage was done to hull, if any, from the dive.
2.the deepest recorded dive by a type VII which survived was 252 meters (820 feet) by U-331, a VIIc, right after it sank HMS Barham.
It appears such deep dives were rare events. Blair mentions two dives by other boats in 41, one to 750 feet (230 meters) and one to 675 feet (208 meters) as "scary" for the crew. Certainly serious enough to be recorded in the boat's log.
The VIIC/41 was supposed to be deeper diving, but Blair never mentions a deep dive in one. Of course, the VIIC/41 only started war patrols in mid-late 1944, when U-boats had a very short life expectancy, so it is entirely possible the record holder never made it back to base.