I once sailed through a river / narrow waterway that ran through a swamp, just passing through, but came upon a spot with 6 crypts right next to each other, right on the water.

It was like Halloween (in more ways than one, I guess)! Load up the ship with iron from one crypt, go across the street, do the same thing...

Crypts also vary tremendously in quality. One that I went through recently had a couple of mud piles. That was it. No blobs. No Draugr. No chests. Nothing else.