I would think waterproofing the seams may very well work. I bet water rolls off the finest available highest grade marine vinyl that they use.
Taken off of the Ultimate Seats webpage:
Foam and covers: (one year warranty)
We manufacture our controlled density memory foam in molds which are built for each and every seat, so the moldings are always exactly the same shape, size and specification.
We use the finest available highest grade marine vinyl for our seat covers, looks and feels like leather but have much better resistance to the weather elements.
A full leather seat is not an available option as we found they do not weather well.
We do not use plastic between our foam and cover like original seats because this holds heat in the cover, our seats offer you a cooler ride.
Our seat cover panels are designed and cut using a computer-controlled state of the art machine that maintains tolerances to half a millimeter.
Cover panels are meticulously hand sewn together by our in house trained skilled sewers.
Our decorative studs are solid brass and chrome plated and will not tarnish.
We also offer many options of colors and embroidered logos for your personalized seat.
We guaranty our seats are the best and back it up with our 14 day trial period.
If you’re not happy for any reason you can return the seat for a full refund.
Enjoy the ride again on your Ultimate seat
Des