The vacuuming aspersion was probably archaeological. Who knows what may have been vacuumed up with the dust: nails, buttons, coins, etc.
So far as the boat degrading faster on land vs in the water, it's the relative availability of oxygen. The ship will oxidize in open air faster than underwater.
Well, I never claimed to be the brightest bulb in the room, but I recall from high-school science classes that water is the ultimate acid, or dissolvent; that nothing can survive being eroded by water, if given enough time.
And so from that, I just assumed a boat submerged in water would deteriorate more quickly than a boat on dry land, but of course, there a multitude of other factors which I did not take into consideration.
One of the fringe benefits of being on the Norway ping list; one learns all sorts of unexpectedly different things, and in a "fun" way.