I should add this one, too, but I just can't wrap my mind around the retail price ($50): the
SealLine Discovery Kayak Seat Cushion.
I received mine as a gift, but it has lasted for *years* without any special care (e.g., I leave it in my fishing vehicle for extended periods). Because it's inflatable, you can modify its firmness & support as desired. Don't know what I'll do when it finally wears out, but I use it all the time now.
Like the inflatable ring above, this is a supplementary type cushion device you'd use with an existing seat that perhaps has inadequate seat padding.
Deus a natibus planis nos liberet / "God deliver us from flat buttocks"
FWIW // joel