It is, basically, rubber. Where leakage occurs the neoprene has been broken, either through a tear or from stitching that was not seam-sealed well or became separated.
I have Wet Okole seat covers that supposedly are waterproof. The neoprene itself is indeed waterproof. But the seats have many seams where pieces were sewn together, and these are NOT factory-seam-sealed. If moisture gets on the seat and sits long enough (or is sat on), it finds its way through these seams. Try it for yourself and see, but put a protective layer underneath first.
One of these days (weeks, months...) I should remove those seat covers and paint a thin stripe of AquaSeal or SeamGrip over every seam, on the underside where it doesn't show. That should prevent any moisture from getting through to the upholstery. A little obsessive, perhaps, but then again it's cheap insurance.
Kayak & Canoe Outriggers
Pull-Up Strap Handle Kit
|Table of Contents|