All of the talk about dead headless ducks and coots in tangled in line, along with the speculation that it is the work of devil worshippers begs the question: How do you catch a coot?? I've never tried, but it seems that it would be pretty hard. I doubt that people are repeatedly catching them for sacrifice without drawing attention to themselves.

IMO this is a die-off from some disease (Newcastle??) or parasite. The line is a mystery, but it could be that they are getting tangled in line on the bottom, and in their diseased condition, they are not able to free themselves. As far as being headless, when a waterfowl dies peacefully on the water, it will float-right-side-up, but the head will sink below water level. Perhaps the head is underwater or was scavenged by small fish. Also, the head will decompose much faster, than the upper body due to being submersed.
NoTrollMotor