But don't lifeguards often work in teams of at least two, frequently (e.g. every 15 - 30 minutes) alternating simply because of that limited attention span?
They do. That's why the OP mentioned rotations and redundancy. Uber could easily put two people in the vehicle and have them swap seats every 20-30 minutes. The person in the driver seat would be responsible for watching the road and reacting while the person in the passenger seat supervises.
It's okay self-driving cars aren't yet single seater.