I think $500 is a little low... I'm guessing $150-$200 a coin x 7 coins, you do the math.. As a clump its more likely to be considered sunken treasure cause its more evident that it is sunken treasure. You say it was loose under the sand, so it could have come from anywhere... I'd work from where you found them out to the depths in a circle pattern.. I'd do your research on the rights to sunken treasure too.... Don't tell anyone where found them and avoid the law until you know the law. You may want to talk to a lawyer.... Are you a diver or do you know how to snorkel ? The chances of finding the place of origin are slim but that should be your primary goal.... Leave that clump alone for awhile and get out there and find some more !! If you stumble across a Sunken Ship... You have a lot to deal with, zoning, licences, rights to the treasure all kinds of things... Also is that Gold in the bottom side of the other pic ??
Keep @ it and HH !!