Most commercial packrafts will be made from TPU-coated nylon, which is a thermoplastic polyurethane-covered fabric that is tough, airtight, heat-sealable, and UV-resistant. Because this material can adhere to itself through heat, this allows manufacturers to create strong bonds without gluing or sewing.
There are some boats on the market, like the Kokopelli XPD, that use PVC, or polyvinyl chloride, as their main material. These fabrics are tough and can resist a hard day on the river, though they will pay for the durability in bulk and weight.