1. The tube should be made of PVC (polyvinyl chloride) rated between 24 and 30 gauge. The higher the number, the thicker the material. If the tube has more PVC exposed to the sun, look for a higher gauge.
2. Air chambers were once thought to provide safety. With the boat on site to retrieve life-jacket-wearing tube riders, multiple chambers just make inflation more difficult. Backrests and accessories may have separate chambers.
3. The cover should be a coated nylon with either a 420- or 840-denier rating. The 840-denier grade is tougher and more UV-resistant.