From July 25, 1946. The first underwater atomic device was detonated at Bikini Atoll, one of the Marshall Islands in the South Pacific. It was the second of two bombs, Able and Baker, that comprised Operation Crossroads; each weapon had a yield equivalent to 23,000 tons of TNT (23 kilotons).
The U.S. Navy conducted the tests to determine the effect of such weapons on ships at sea.More than 130 newspaper, magazine and radio correspondents from seven nations were present for the tests. (http://www.peacebuttons.info/E-News/peacehistoryjuly.htm)