I second that diving alone is OK. I got 'qualified' to dive from a college course before they had certifications. One has to be not stupid. I dove every Sunday for about 5 years in Puget Sound with 4 other divers. Visibility sometimes was 3' and other times it was 15'. Either way, it only takes about two kicks to be out of someones visual range. It seemed like we spent all our time trying to keep track of each other. I too use a pony tank and spare regulator. I retrospect, it was probably a good idea to try to stay together when we first started diving but after awhile it was senseless. I dove alone for years on Lake Washington, with SCUBA, working an underwater metal detector. I rarely found much stuff over waist deep and I figured if I could stand up where I was diving, I was relatively safe. I think other things, like how you respond to critical situations you have encountered in life, are another important predictor of what you will do when it gets hairy. For example, if you are a rock climber or traverse glaciers, or sky dive, work high rise construction, are a law enforcement officer or a fire fighter, etc., and get practice and rehearse emergency action to keep yourself safe in other environments you probably have an edge becoming a safe solo diver. On the other hand if you live a sedentary life with little exposure to dangerous situations, you probably ought to dive with somebody all the time