I use the throwable Type 1 PFD cushions and two sport-a-seats (the most comfortable on the boat).

My dog needs a longer cushion to stretch out so we use a Coleman Cooler Cushion for him. Roughly 15" or 16" wide by 35" for under $50.You can get other sizes larger and smaller. We try to keep it dry because it's open cell wrapped in plastic construction -- but we really don't worry too much about that cushion.