unless you are playing some kind of hardcore mode then I strongly suggest using the seed viewer online to find a good swamp. Iron is the most needed mineral in the game by far and unfortunately, there are loads of swamps on every map that have no crypts. some people look thru upwards of 10 swamps before they find one with crypts. that's how it is. lots of tiny, empty swamps and a few large swamps FULL of crypts. I spend a lot of time looking at random seeds on the viewer and I've noticed that this has only gotten worse after the mistlands update. Mistlands areas are MUCH larger than before and this has the effect of squeezing everything in toward the middle because the mistlands are all around the edges of the map. swamps were small in comparison to begin with and they are even smaller now.
given all this, I personally don't feel bad looking online for a decent swamp.