If you're looking for a souvenir that truly opitimizes the adventure of the Key's shipwreck history and don't mind something that is a bit on the pricey side, you have to check out the Treasure Chest.
This shop is full of all sorts of treasure that has been recovered from a variety of old, and often historically significant, ships that wrecked off the coast of the Keys. With prices ranging from approx $100 to "anything you've got", this business offers all sorts of pieces, from coins, to jewelry, to an assortment of old trinkets. The pieces are from a number of different time periods, many dating back beyond the 17th century.
Best of all, the history and adventure of the old wrecks is nicely set-out by the proprietors, who are sometimes the same people who dove on the wrecks. If you're lucky enough to speak with some of these people, they'll be happy to chat with you about their own experience, which is quite exhilerating. The stories surrounding some of the wrecks are fascinating.
We ended up getting a 1770 coin from the El Cazador and a pair of silver Spanish coins from the Atocha (that were made into cuff links).
I can't speak for collectors, but, as a layman in such matters, I can say my girlfriend and I truly enjoyed the experience and the shopping.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.