What and where are "flotation seat cushions"? Inspired by the question Why did the US Airways Flight 1549 passengers stay on the wings?
I read parts of the respective accident report[PDF]. They mention the flight was a
non-EOW (extended overwater) flight but was luckily equipped as such anyways and thus had life vests and 
"flotation seat cushions". 

Section 2.10.3.3.1 "Equipage" of the report states


  Because the accident airplane was equipped for EOW operations, 
  it carried life vests for both passengers and crewmembers. 
  However, given that the accident flight route was not an EOW 
  operation, the airplane could very well have been equipped 
  with only slides and flotation seat cushions as the primary 
  means for passenger flotation.


I've never heard of such a device, I only know the life vests below a
passenger's seat. Can the seat cushion (the part where you sit on, I assume)
be detached from the seat to serve as a "swimming aid"?

Maybe it's different in the US and Europe. I sometimes fly from Germany to the UK
and the FAs never mentioned any cushions. How comes?
It became something of a meme:


  In the unlikely event of a water landing, your seat cushion can be used as a flotation device.


You remove your seat cushion and hold the straps on the back as a flotation device. It's probably more of an aircraft/airline thing than by country. It seems that the industry is favoring life vests more, as they are designed to keep a person afloat and face up even if unconscious and not adding the requirements to the cushion allows the seat to be smaller and lighter.



Source