Originally posted by MeaNGremmy:

Above comments are correct. Also sailing further away from the center seems to increase the chances of finding swamps with plenty of crypts. At least according to some anecdotes.

Not true in my case.
I play on a server with 9 friends so I wanted to find 'fresh' swamps and started near the edge of the world. Still no crypts in the first 5 swamps I found.