The Type VII was an outstading design.
At a time when American boats were made with riveted construction, the Type VII used all welded hulls with modular construction.
The high quality steel used in the pressure hull allowed diving depths that exceeded anything the Allies had, even at the end of the war.
At the early part of the war, US boats had max depth of 300 feet, at the end of the war, had increased that depth to 450 feet through by adopting welded construction and higher tensile steel.
The Type VII routinely dived to 600 feet, and as described in the book (and movie) DAS BOOT, was able to dive to 300 meters/1000 feet in emergencies.
Post-war Type VIIs were studied intensively by US and Soviet navies.
For the US, that led to the Tench class diesel boats, and to the Russian Foxtrot class diesel boats.
In the submarine community, DAS BOOT is regarded as THE most realistic submarine movie yet.
I read the book while at sea years before the movie was made. The book has photos of that war patrol.
There's also a companion book called U-Boot Wars that documents the next ar patrol by the author, Lothar Gunther Buchheim.
While the Type VII was a great design, the boat could have turned the tide for Germany was the Type XXI, which the most advanced submarine in the world at that time.
The Type XXI was a superlative design which was built for underwater speed and endurance.
Faster than any destroyer, and able to stay submerged for days, it could have inflicted huge damage on Allied fleets.
Fortunately, like many of Germany's wonder projects, it came too late.
Here's a link to the only surviving Type XXI, which is in Stockholm.
The Type XXI was the pattern for all post-war submarines up to 1953.
Up to that point, all submarines (non-nuclear and nuclear) used a variation of the Type XXI hull form.
Even the USS Nautilus, and the Skate class looked similar externally to the Type XXI.
It was only after 1952, with the development of the USS Albacore, did the US and eventually the world, shift to the now common modified teardrop hull.
As far as submarine design, I rate the Type XXI as:
1) A decisive paradigm design shift towards major underwater performance as opposed to mixed surface/submerged operation as with the US during WWII.
The next major design leaps are:
2) Nuclear propulsion (USS Nautilus),
3) The teardrop hull (USS Albacore research submarine, and USS Barbel class, which was the first warship using the teardrop hull, though diesel-electric),
4) The marriage of the pure teardrop hull with nuclear propulsion (USS Skipjack), which had speed and maneuverability that was breathtaking, and held the Atlantic crossing speed record by any vessel for over 20 years.
5) Modified teardrop hull, nuclear propulsion, much greater diving depths, and spherical bow sonar array with mid-mounted torpedo tubes (USS Tullibee and Thresher). A more practical design than the pure teardrop hull like the Skipjack and easier to construct. All US submarines are based on this basic design.