Contrary to what would be expected, it's not the swords or the armor unless you find a field absolutely riddled with them. It's the ruined bridges that you find every now and then that give the most iron, and you don't even need to waste time and coal smelting it.

If you can get 2 pieces of iron and 6 of copper from the swords/armor, you can build a forge/stonecutter beneath a bridge and just break them with a hammer. Then you just wait for the rest to fall down, reap the resources and load them on the ship.

Hope this helps someone, cause i just got 3 stacks of iron and 6 of marble, with next to zero effort and way more enjoyment than crypt hunting.