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.
In 1918 the San Diego was assigned to convoy escort duty, and on July 19th was holed below the waterline while zig-zagging on a westerly course off of Fire Island, NY. The San Diego sank in 28 minutes with a loss of six crew men. Although the original conclusion was that the ship had been torpedoed, later investigation (and subsequent wreck-diving) decided that it was more likely that the damage was caused by a mine. The San Diego was the largest American warship lost during WW1. You can read more about the German mine-laying here.