The U.S. Navy became aware that the range of certain U-Boats included the U.S. East Coast in October 1916. You can read about U-53’s unexpected visit to Narragansett Bay here.
Laid down in 1902 the U.S.S. San Diego (ACR-6), originally the U.S.S. California, was one of a class of six vessels intended to be battleships but the launch of HMS Dreadnought in 1906 rendered them obsolescent in that role so they were reclassified as a type of cruiser. You can read more about the San Diego here.