Germany lacked the industry to build a surface navy equal to the British Fleet as well as undertake their other military programs.

The British Gross Domestic Product, a very crude measure of economic size, was roughly equivalent to the German in 1938. https://en.wikipedia.org/wiki/Military_production_during_World_War_II#GDP

However Germany had to focus on their land forces, due to their geographical position, and the German navy was funded at a much lower level than the British navy. Additionally, large warship building in the mid-twentieth century took years to complete, and the British had a 20 year headstart in their ship building program. Britain would have reacted to any German program with an increase in their own production, and it would be very unlikely for the Germans to ever gain naval superiority.

Lastly, it should be realized that the Nazi foreign policy, while militaristic and aggressive, sought to avoid conflict with Britain and seek expansion in the east, so a naval build up would not have been a sensible investment. The 1939 attempted intervention of the Western Allies in the German invasion of Poland came as a surprise to the Nazi leadership.

German shipbuilding efforts were just not up to the job. They had lost what knowhow was left after WW1, and tried their best to catch up 15 years later with at best mixed success. While you can pump out a few aircraft a day, even small ships like subs or destroyers take months or ideally the best part of a year before they are useful between building and fitting out. While the limited German ship building industry that remained, and smaller KM budget, along with even greater strain on the propped up "imaginative" German prewar economy precluded any real expansion unless it might come at the detraction from either of the other branches.

The RN built more warships of every size and in general of at least the same if not better quality than the Germans. While if Germany redirected resources from her capital ships to small quicker to build small combatants or even subs it also consequently allows the RN to face an even more one dimensional threat. And even a doubling of the early war U-Boat threat doesnt pass the smell test as a war winner. In the 28 months between the start of the war and the entry of the US, the KM sank enough shipping to just outpace new construction(not even really bring about starvation) just 4 times, at even then with losses amounting to nearly the total number of U-Boats that had been in service when the war began.

I would suggest this WarCollege thread where we break down some more of the numbers. https://www.reddit.com/r/WarCollege/comments/56ueh0/could_uboats_have_turned_the_war/