100 war-ready Type VII U-Boots at krieg's begin. That would have done it. Germany had around a dozen Type VII's ready for the Battle of the Atlantic in 39. Doenitz said he needed 300 to be successful. I think, realistically, Germany could and should have had around 100 ready at war's begin (some hidden because of T of Versailles), with plans to produce a further 200 within 2 years. A massive concentration of wolf packs, before suitable counter-measures had taken effect, would have spelt DOOM with a capital D for the UK. It was only in mid-42 when the Allies finally had some decent weapons to use against the U-Boats, and aircraft finally had long enough range to cover the gaps. If we combine this, with another what if and say that the Germans developed the schnorkel 2 years earlier, well....

Germany's leadership squandered the little naval resources and potential they had. They should have learned from WW1.