Lots of good info above, however the question you should be asking is, "What is a good PFD for a rafter who plans to boat ___ type of water?" Then questions about adjustments to the frame, rowing position, what kind of seat, etc., come after that.
Pick your PFD based on what will bring you back to the surface of the water and work best for the use you'll have, not based on what kind of seat you'll have.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse