7 September 1776.
On the night of September 7, 1776, in support of the upcoming Battle of Kip's Bay, Turtle, under the guidance of army volunteer Sergeant Ezra Lee, attacked Admiral Howe's flagship HMS Eagle, which was moored off what is today called Governors Island, which is due south of Manhattan.
http://en.wikipedia.org/wiki/Turtle_(submarine)
While the attack was unsuccessful, it was considered the first attack by a submarine on a warship.
Be more aggressive!