When you say the "water's edge", do you mean at the salt water/ocean beach? If so, you may just have a chunk of typical cr*p typically found on wet-salt-beaches. Give enough time for a chunk of iron or ship-part junk to fossilize sand and corral around it, and you get all sorts of wierd metalic junk that is tumbled around in the surf for years. Here in CA, we un-fondly call such junk: "submarine parts" and promptly throw them back into the ocean