No need to bring one. They will tell you "as we will be flying over water for a portion of this flight your seat cushion becomes a flotation device in the event of a water landing."
So now we have established that it is a PFD and they have identified it as "YOUR" seat . . . so take it with you when you get off the plane.
Problem solved. ;-)
Charlie P. P31-2 #80 CATNAP
To travel hopefully is a better thing than to arrive - R.L. Stevenson
I suspect that, if you should go to the end of the world, you would find somebody there going farther . . . - H.D. Thoreau