Not the same hobby, but I watched a lad from tarn doing that magnet fishing, pulled out a gun down darton longfields. ANother fella on youtube, in the canals of london, allsorts including hooking onto a safe which he had fun getting out. Must be all sorts of stuff thrown in rivers and canals, or lost through history maybe where armies have fought or crossed. Plus a few shopping trolleys.
Hey I watched this show on the Beeb this summer, about the metal minded scrap scrapers lol. Seriously though, I enjoyed it, and it made me realise that it's big business, its a serious thing to the collectors.