There are a couple of brands of water-filled seats you can use. "Bun-eze" is one in particular. It is pretty large and comfortable...however, the suction cups wont stick to textured surfaces and it is filled with some kind of foam/fiber that makes it kind of difficult to get all of the air out. (you float)
Another brand we carry is specifically called "Water Seat". It doesn't have the suction cups (really kinda useless anyway) and is more of a simple bladder design (no foam) Fairly large as well and much easier to fill.
Hope this helps