It seems like, rather than trying to create a counter-blockade of the British Isles (and provoking the United States in the process), Germany might have been better served by attacking the British Navy itself.
Apologies if there's an answer somewhere on the subreddit already. I found questions about the blockade, but no answer to this particular question.
The German navy did attack the British fleet with submarines from time to time, but the British blockade of Germany was what's called a "distant blockade." Essentially, the main part of the British fleet was based in Scapa Flow, at the far northern end of Britain, and cruisers and smaller ships guarded the approaches around the Flow in between the UK and Norway. The English channel was guarded by its narrowness (you can see right across) and friendly forces controlled both sides of it. So the only possible place for supplies to reach Germany was from the north of Britain, through the UK-Norway gap. The British fleet patrolled and controlled those approaches, but the cruisers and destroyers it used to do that were hard for the German submarines to find, and the main part of the Fleet stayed inside the Flow, which had installed U-boat and torpedo barriers at the various entrances. It wasn't the type of blockade in earlier wars, where ships would stand off the entrance of a port to watch.
The Germans actually built so called merchant submarines and the most famous one and that also made a trip to the US and back was the submarine "Deutschland". Built in 1915 it could carry 700 tons and was mostly used to transport important rare materials for german industries like rubber. It was also a big PR coup for germany and garnered huge sympathies when it reached the US east coast. The UK/France promptly protested the use of the merchant submarines docking and trading with the US since they could not be inspected nor stopped like normal surface vessels. The US rejected that argument though.
The ship was later converted into U-155 sinking 42 merchant ships or 122.000 gross register tons. She finally returned in November 1918 and was later that month surrendered to the Entente and eventually scraped.
Uboat.net has a short info page about it aswell as wikipedia. Gibson´s The German Submarine War, 1914-1918 touches on the subject aswell
Edit: Oh boy, seems like I missunderstood your question. Still since it somewhat fits I will leave it here
/u/jschooltiger and /u/disgruntledhobgoblin both have good answers. Here's my contribution.
Until late in the second world war, submarines were little more than mobile sea mines. Their submerged speed was extremely low and even on the surface they were outmatched by most warships. The best they could do was to position themselves at points they were likely to encounter enemy ships and wait.