Maybe have the shipwrecks gather one new item like pirates treasure or something and then have the value of each shipwrecks treasure be randomly generated within a certain range and unknown to the player until it is sold. So someone might find a lucky ship with the max payout. Obviously not some ridiculous amount since it will be there all restart but small variations would be cool. Sell it wherever other precious materials can be sold i.e. silver, diamond, iron etc. Not sure if its hard to add different values to the same item but gathered from different wrecks but would be cool. Pretty much first idea but variations from ship to ship of item price.