Underwater scooters also known as diver propulsion vehicle, is an item used in scuba diving mostly which helps divers to use less air. The amount of air consumed mostly depends on the calories you burn underwater. So, this simply means you go faster, you breathe more. Logical isn't it? But this is the last thing a scuba diver would want since they have restricted amount of air in their tank. So, it seems better to consume minimum air in order to be underwater as much as possible.
To achieve the maximum time underwater depends on how relax you are while diving. Of course you can do it by getting some experience. However, in any cases you should use your legs to give your fins the propulsion force, doesn't matter how much experience you have here. Instead of using your legs, it is an alternative to use underwater scooter. It gives you the feeling of smooth diving.
Don't hesitate to try it. I know it seems a bit muscular but it is very silent indeed and it does not do much speed underwater, it imitates your fins speed.