I found my 1st crypt on the very edge of the water. Dropped a workbench and used about 200 stone to make a pier out into the water and a small island for a shack with a portal and some chests (not a base!). Dock the boat on the backside. For some reason the leaches would spawn around the boat but not touch it.

There were 7 crypts within reach of that little camp, so I thought it was worth the effort.

And yes. Many swamp areas are too small, thin, or just plain don't have crypts. Getting a little farther out from the spawn stones does seem to help. I probably found a dozen empty swamps before I found a crypt. Most were very small.