According to this article, Germany was beginning to realize that they would not be able to conquer the sea using submarine warfare. These claims were based on the fact that German newspapers were no longer boasting about the military and naval power of Germany. Combined with the growth of the British navy, German critics were realizing that submarines would not dominate maritime warfare as they had planned.
“German Critics Beginning to Realize that their Submarines are not likely to Conquer the Sea,” Berlin Daily Telegraph, June 30, 1915.