The "preserve it for the enjoyment of everyone to come" arguement is lovely, if you're talking about wrecks that will be around so long as no one disturbs them. Here in New England, those wrecks that aren't swallowed whole by the churning sands, dashed to bits (of sea trash) upon the jagged rocks, or twisted and dispersed by storms generally either break apart and deteriorate over a relatively short period of time, or become so encrusted with a hard mineral conglomerate that you'd need an air hammer to be able to tell a horseshoe from a musket.
Leaving all the artifacts on these wrecks is the surest way to see that no one ever gets to enjoy them except for an occasional fortunate trawler fisherman or beach comber. Not that I'm saying you should strip every wreck you come across, but I don't at all have a problem with looking for a cool souvineir or two.
In the brief history of European inhabitance, there have been thousands of shipwrecks off Cape Cod alone, mostly not particularly significant ones. A site listing good dive sites off Cape Cod identified the known locations of about 5 of them.
The Massachusetts Board of Underwater Archaeological resources
http://www.state.ma.us/czm/buares.htm lists a whole thirty eight known wrecks for divers in the entire state, and there are less than a dozen wrecks for which reconnisance and salvage permits have been issued.
The remainder are not just sitting forgotten in pristine condition on the bottom... they're mostly gone.
The official position on artifact recovery here is this:
Generally, those resources are defined as abandoned property, artifacts, treasure troves, and shipwrecks which have remained unclaimed for over 100 years, or exceed a value of $5,000, or are judged by the Board to be of historical value. The Commonwealth holds title to these resources and retains regulatory authority over their use. The Board's jurisdiction extends over the inland and coastal waters of the state.
If you find it first, and apply for the proper permits, the state keeps a third.
