Is the extra liability the reason you don't rent scooters?
No, that's not it. I don't rent my scooters for several reasons. Mainly because of the way people treat rental equipment. There's a common phrase you will hear often around here, and to be polite I will quote it as: "Screw it, it's a rental!"
I hear this all the time when folks are talking about their rental cars / jeeps and other things that they have rented. I've worked for dive shops that rent gear and I can't even begin to count the times that folks bring the gear back and it's just simply trashed (flooded, broken, sandy, missing parts, etc., etc.). I wouldn't want that to happen to my life support stuff and I wouldn't let that happen to something that I would then rent to another person to go out and take chances with.
The scooters are expensive enough for me to want to be there first hand if there are any problems, leaks, short circuits, etc. Also, I love scooter diving so I like to get out on them every chance I get!
I also don't rent any other equipment either. I don't rent regs, BC's. All necessary equipment is included in my tours and one way I'm confident that it's working up to par is to not let it out of my sight.
Finally, I don't think I've ever met a diver with a DPV certification. I know they exist but I never met one. Without this 'proof' that you know how to scooter dive I would question whether or not you were truly capable of taking them out safely. There are certainly some extra safety and bouyancy concerns when you add scooters. Then there's always the environment. I've seen people crash the scooters into corals, and I've seen them blast up tons of sand and I've seen them drop the scooters on top of living things. That's not good and so as a matter of principal, I just don't rent them to strangers. Even when I am escorting people on Scooters they sometimes do 'bad things' with them and it makes me cringe. All I can do is try to educate them and point out what is not right and help them learn the proper way. Something that is easier said than done!
If you're coming to Hawaii and you want to scooter dive, let a pro take you and you will get much more out of your experience than if you rented and went on your own. It will also be a lot easier in the long run.