For decades, the U.S. Navy has trained against Australian diesel-electric subs, and often came out second. The Gotland has one advantage over the Australian boats, because of its AIP system (which allows it to stay under water, silently, for several weeks at a time). Thus the Gotland is something of a worst case in terms of what American surface ships and submarines might have to face in a future naval war. None of America's most likely naval opponents (China, North Korea or Iran), have AIP boats, but they do have plenty of diesel-electric subs which, in the hands of skilled crews, can be pretty deadly.
It's only a matter of time before China gets lots of AIP boats. Meanwhile, China is rapidly acquiring advanced submarine building capabilities. One new Chinese diesel electric sub design, the Type 39A, or Yuan class, looks just like the Russian Kilo class. In the late 1990s, the Chinese began ordering Russian Kilo class subs, then one of the latest diesel-electric design available. Russia was selling new Kilos for about $200 million each, which is about half the price other Western nations sell similar boats for. The Kilos weigh 2,300 tons (surface displacement), have six torpedo tubes and a crew of 57. They are quiet, and can travel about 700 kilometers under water at a quiet speed of about five kilometers an hour. Kilos carry 18 torpedoes or SS-N-27 anti-ship missiles (with a range of 300 kilometers and launched underwater from the torpedo tubes.) The combination of quietness and cruise missiles makes Kilo very dangerous to American carriers. North Korea and Iran have also bought Kilos.
The Chinese have already built two Yuans, the second one an improvement on the first. These two boats have been at sea to try out the technology that was pilfered from the Russians. A third Yuan is under construction, and it also appears to be a bit different from the first two. The first Yuan appeared to be a copy of the early model Kilo (the model 877), while the second Yuan (referred to as a Type 39B) appeared to copy the late Kilos (model 636). The third Yuan may end up being a further evolution, or Type 39C.
The Type 39, or Song class, also had the teardrop shaped hull, but was based on the predecessor of the Kilo, the Romeo class. The Type 39A was thought to be just an improved Song, but on closer examination, especially by the Russians, it looked like a clone of the Kilos. The Yaun class also have AIP (Air Independent Propulsion), which allows non-nuclear boats to stay underwater for days at a time. China currently has Song class, 12 Kilo class, one Yuan class and 32 Romeo class boats.
Based on the experience with Australian and Swedish subs, the U.S. Navy has been developing new anti-submarine tactics and equipment. In secret, obviously. But now there is another complication. Animal rights groups have succeeded in getting courts to rule that the navy use of their anti-submarine training center near San Diego is illegal, and use of sonar must be limited (because of possible harm to marine animals). The navy keeps fighting the lawsuits, but it now faced with the prospect of abandoning the training site, for one somewhere the lawsuits can't reach. The training site is expensive because it is wired, with under water sensors that enable training or test results to be precisely recorded, and changes made to equipment or tactics. The center also enables sonar operators to get realistic training. There is no opportunity to get trained on the job, as mistakes can get your ship quickly sunk.
Meanwhile, potential enemies build more of their cheaper, and higher quality, diesel-electric boats, and train their crews by having them stalk actual warships (including U.S. ones.) The subs are getting more numerous, while U.S. defenses are limping along because of the sheer technical problems of finding quiet diesel-electric boats in coastal waters, and the inability to train and test enough because of lawsuits.