I've seen that xkcd before. Maybe I made the wrong association because so far most wrecks which have been used for that were in deeper waters. Which when one thinks about it probably is for the simple reason that there were more fights at sea, than in rivers.
The most famous collection of wrecks used for this, at Scapa Flow, are in 12 to 45 metres of water [0]. Not really deep, as the ocean goes.
Generally the ships are deep enough that it was uneconomic to salvage them before, but not so deep that it's prohibitive to get at them now.
The USS Indiana, also mentioned in the article as a source of low-background steel, was never sunk [1]. It seems it was dismantled and meant to be scrapped, but no one got around to recycling the steel.
