A traditional word for those who salvage on the sea is "wrecker":
It's evocative of a time when salvaging wasn't considered a very nice thing to do.
The used equipment business I've been involved in requires that we go to auction and buy equipment at the cheapest possible prices. One of the groups of people you meet at these auctions are called "scrappers'. They purchase items at auction for metal (and plastic and perhaps other) scrap value. Some of them own their own scrap yards but we might buy something that we know we can scrap.
Anyway, scrappers sometimes are on the wrong side of the law in that if you leave a thing around long enough, which has enough scrap value, and the thing is in a region which is sufficiently unobserved, then eventually a scrapper will come along, take it up, and sell it to a scrap yard. I've spent time at night watching for these folks and have surprised them a few times. In this sense, "scrapper" means "scrap metal thieves".
A typical object for scrappers is manhole covers. For a use of the word see:
"A scrapper might get about $20 covers and grates, but it can cost the city more than $200 to replace a single manhole cover. A few years ago, scrappers got only $35 a ton compared to the current $425 price for a ton."