" During its first year in the United States, the Gotland conducted approximately 160 training days at sea, supporting strike groups, individual ships and rescue submarines, as well as participating in testing and development of new equipment. This included acting as an opposing force so that the United States could practice anti-submarine warfare (ASW) capabilities, especially against smaller submarines operating close to the coast. Reportedly, during a Joint Task Force Exercise on 6-16 December 2005 with the USS Ronald Reagan Carrier Strike Group off the coast of Southern California, the Gotland managed to take several pictures of the Ronald Reagan from close quarters, indicating a "strike" on the aircraft carrier.The submarine evaded detection so effectively that the United States requested a 12-month extension on its lease, in order to continue performing the joint exercises. In July 2007, the HMS Gotland left San Diego to return to Sweden.
"To put it simply, if naval exercises in the last two decades involving foreign diesel-electric submarines had been actual combat, most if not all, U.S. aircraft carriers would be at the bottom of the ocean: as many as 10 U.S. aircraft carriers have been reported “sunk” in these exercises.
Ships in exercises are lost all the time. It’s the military practicing... what happens in “... scenario”.
The Navy has a great track record given all ships that have come in and left.