I was pretty lucky in that I found a small swamp near my main island with a single crypt, but that gave me enough iron to make an iron pickaxe and a longboat. With both of these, I sailed across the seas looking for another swamp and after a few hours I found a pretty big swamp with 9 crypts... they were in every direction! After visiting just 4 crypts I managed to get like 500 iron, I hauled it all on my longboat and took it home to smelt it over several days, it wasn't too bad.