I had the same problem at first. I sailed Northwest of my starting area I found 3 or 4 swamps with only 1 Sunken Crypt among them. Twas very frustrating, but like a Zep Tepi posted, those swamps were all thin strips of swamp along the edge of Black Forest biomes. I then sailed Southeast of my starting area and the first landmass I came across had 2 very large swamp biomes separated by a strip of Black Forest. I sailed completely around the land mass and on the Southeast corner found an abandoned Draugr village in a meadow. I set up a headquarters there as it already had 5 buildings that needed only a little repair. Between the 2 swamps I found a 9 Sunken Crypts and ended up refining all my ore at my 2nd base.