At the outbreak of war, the Kriegsmarine had a relatively small fleet of 57 submarines
It took the allies until middle of 1943 to gain an upper hand against the German u-boats. The initial stages of the Battle of the Atlantic were spectacular successes for Germany in spite of initially a very small u-boat fleet.
Intuitively it seems likely that had Germany started the war with more submarines the War in the Atlantic might have gone very differently and potentially caused Great Britain to have been successfully blockaded. The slower numbers allowed allied forces more time to change tactics and develop counter warfare tactics that might not have been allowed had Germany had considerably more submarines initially.
What would have been the impact on World War Two had Germany focused more of their naval production on u-boats prior to WWII?