Submarine production was not among Hitler's priorities and that was one of his many strategic mistakes.If indeed the Allies were to face a significant U-boat force, instead of the few dozen operational submarines at a time, the outcome of the war could have been different. There were 19 shipyards producing over 1100 U-Boats from 25 operational variants over the period of 1934 to 1945.

From 11 cities and 19 shipyards based along the North Sea and the Baltic, Germany turned out more than 1000 U-Boats. 103 where built between 1935-1940 and a staggering 1050 from 1941 to the end of the war. The cities were: Bremen, Hamburg, Kiel, Lubeck, Stettin, Danzig, Flensburg, Vegesack, Rostock, Wilhelmshaven and Emden. These shipyards built a total of 1,153 u-boats from 1935-1945.

Incomplete German U-boats abandoned at the Blohm and Voss shipyard in Hamburg, 4 May 1945 [BU 5290 from the collections of the Imperial War Museums] 

Type XVIIB U-1406, partially dismantled shortly after the end of World War II [US goverment employee]

Unfinished German Type XXI U-Boats at the AG Weser shipyard in Bremen, Germany, 1945 [iliketowastemytime.com]

Unfinished German type XXI submarines on the slip No. 4 shipyard AG Weser in Bremen April 1945. U-3060, and U-3062, foreground, from left to right — U-3061-3063 [Ronnie Bell (Flickr)]

Unfinished German type XXI submarines on the slip No. 5 shipyard AG Weser in Bremen. right lane U-3052, U-3042, U-3048 and U-3056, the left lane the U-U-3043, 3053, U-3049 and U-3057 April 1945 [Ronnie Bell (Flickr)]

The Blohm and Voss slips in May 1945 give an idea of the level to which series production of submarines had progressed. These Type XXI boats are being assembled from hull sections prefabricated at many inland sites and transported to Hamburg by water [ww2-weapons]

U-boat Production: Three Type XXI submarines on a building slip at Hamburg right. After Germany surrender [carridisvenforum.altervista.org] 

U-Boat Type XXI on the AG Weser slipway 1945 [bunker hornisse]

Scene of destruction in the Blohm-Voss shipyard, Hamburg, Germany. The remains of German midget submarines lie in the foreground [histomil.com]

Two wrecked German submarines at dockside in Hamburg, Germany, after Allied bombing. Shipyards were rebuilt a few times, but after the heavy bombing in April 1945, they were abandoned [historylink101.com]

Bremen Shipyards after an Allied raid [www.626-squadron.co.uk]

Allied reconnaissance photograph of Hamburg, 1945 [www.nationalarchives.gov.uk]

Type XXI U-Boat U-3008, Wilhelmshaven, Germany June 1945 [www.militaryfactory.com]

By war’s end in mid 1945, German U-Boats had sunk ≈ 3000 Allied ships, less than 5% of the ships built during the war, only one of them a loaded troop transport. 783 of 1170 U-Boats launched had been sunk, mostly by American, British & Canadian forces. Of about 37,000 Germans who went to sea in U-Boats, about 28,000 never returned. But first they killed over 40,000 Allied seamen among whom only a few thousand were naval personnel.