Yikes a really good thread,
As a semi-comercial diver for sea urchins I dive solo every dive. There are 4 divers on the boat and we work as a team to locate the urchins and understand the conditions we dive in but when it comes to splash time you are a completely self contained unit, alone in the water and better be ready for whatever happens to you. The other divers may be in the water close to you but when it all falls apart you had better be able to deal with it yourself as they will be out of range or have their heads down working and making noise enough to cover anything you may attempt to do to contact them.
I have been diving since the mid 70's and solo diving happens to the best of buddies quite often. You start with a plan with a buddy, get seperated and all of a sudden your alone. This is in many respects solo diving. A real example might be if you happened to get caught in a monofilament net on a wreck just as you go to make your assent, and you have lost your buddy, you are in a world of trouble if you are unprepared to do a self rescue. Your buddy also low on air will probably follow your plan and after not locating you begin his assent, as planned! Low on air, and caught in a net. A pretty scary proposition unless you are prepared. Redundant knives, a pony that is full, and enough experience to remain calm and focused on what you will do to get back to the huge air tank at the surface. The experience part is very important. It may come with 50 dives or a 100, you may dive the first time ever and be ready to do solo diving, most of it is in how your brain reacts to very stressfull situations. Some very good divers do not have the apptitude to solo dive as they do not have the ability to think clearly when stressed to the max.
I have been in some scary situations alone. Nothing immediately life threatening but complicated and stressfull. Entanglements happen easily, o-rings fail sometimes, equipment breaks and you get a very narrow focus and forget your other "obligations" and find yourself very skinny on air.
I could go on and on but I enjoy solo diving. It has its risks but it has its freedoms as well. you get to focus on what you are doing, you are responsible for only yourself, and you do not need to worry about the other guy.
Just as an aside I dive in the north Atlantic off the coast of Nova Scotia in the winter. Water temps drop to 32 in February. We deal with a completely different set of conditions than most as everything and I mean everything is affected by the cold, your gear, your body, your brain.
Cheers and Happy New Year
Sandy