Visited the Duke a few times, and I'm pretty sure it was in concrete, Incognito,( unless you know better
) the thing isn't just resting upright in estuary sediment, it wouldn't have been safe enough if it was being boarded by people regularly, and with all that weight would have shifted and tilted after all these years. down in the oily depths of the ship it's clear that the castings surrounding the rudder housing are cracked severely, maybe irreparably,so i doubt it could ever be seaworthy again, without a lot of money thrown at it.
it would be good to hope that it may be used for some useful purpose in the future whereby those who are interested can get to see the wonderful 'frozen in time' nature of it, the passenger seating area, with 70's Sealink posters still on the wall, and that very retro looking bar area featured in the coast programme
http://www.bbc.co.uk/iplayer/episode/b012l0h7/Coast_Series_6_Wales_Border_to_Border/
(The Duke of Lancaster part stars at around 53 mins in)