Fish traps, they hang nets between the poles, very nasty if you get
caught in one. Unfortunately they are all over the Chesapeake, mostly
in the shallows and off the beaten track.

All over? Not quite. I've only seen about 6 of them total, at least out in
the Bay itself. They are usually in waters less than 10 feet, a hundred
yards or more offshore. And they have a tendency to not show up reliably on
radar...