Flags on sunken naval vessels

The wreck of the Royal Oak is now an official war grave and there is an annual commemoration service. Royal Navy divers go down to the ship and raise a White Ensign in memory of the 833 crew killed.
André Coutanche, 15 August 2005

http://www.navweaps.com/index_inro/INRO_Repulse.htm recounts the loss of warships Repulse and Prince of Wales but also shows, as the second photo on that page, a flag under water.

Caption: "The White Ensign attached to the port inner shaft of the HMS Repulse, ninety feet (30 m) down in the South China Sea."
Jan Mertens, 12 August 2005

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The flagpole is attached to the (wreck of the) USS Arizona herself, whereas the Memorial is shaped like a bridge, touching neither the wreck nor the flagpole.
Miles Li, 17 August 2005

Independent Television News (ITN) reported yesterday evening, 21 May 2007, on the raising of a White Ensign under the sea, in Falkland Sound above the wreck of Her Majesty's Ship Ardent and the placing of two plaques on the wreck:

"FOR THOSE THAT LOST THEIR LIVES ONBOARD HMS ARDENT FOR THE FREEDOM OF THE FALKLAND ISLANDS 21 May 1982"

during the landing of British forces on the islands in the Falklands War.

However, note that the photograph of the plaque which appears on the ITN web site states that the plaque was laid on 25 January 2007.

Sources:
(1) ITV News and Weather, Independent Television News, as broadcast UK terrestrial television channel ITV1, Monday 21 May 2007 commencing 22:30
(2) Independent Television News web site, <http://itn.co.uk/news/c39febdd8299240136fb45ed6e3f4e16.html>, stated to be updated 15:21 Monday May 21 2007 and consulted 22 May 2007
Colin Dobson, 21 May 2007