My first real scooter (after my self-collapsing Tekna) was a Silent Submersion or "SS" scooter. I've never had one stop on me for any reason, and I've never looked back. I now have three (soon to be four) and instead of replacing them, I upgrade batteries and keep diving them. When the survey front is 15,000 feet from the entrance, you don't want to wonder if one of your scooters in going to fail and make you swim, or worse. Silent Submersion scooters are easy peace of mind. You can check out their webpage here: http://www.silent-submersion.com/
Two Silent Submersion Vipers, ready to go to 9000 feet to survey and explore at Jasper Blue Spring.
Two Silent Submersion Vipers, back from 13,000 feet and the end of the line at Cow Crap Cave.