Thanks for the input so far. A very good list of "Do's and Don'ts" has been generated.
If any are still reading this thread, I'd like to kick the discussion slightly sideways with another question:
For the solo diving you do, what do you consider the most dangerous aspect of the dive?
Honestly, the most dangerous aspects of a solo dive is driving to and from the dive site or the boat, boating, showering before or after diving, and doing all the normal activities people do each day. The most dangerous sport, statistically, is fishing if MANswers is correct. We find ourselves more likely to be injured in and around our homes. We spend more time around our homes and haunts and are more complacent about safety. For example, I nearly slipped several times in my girlfriend's shower before going to teach diving or after a dive. Eventually, I mentioned we'll have to put a non-slip surface in there.
For those who dive often, our home waters, caves, wrecks and haunts become our second homes. We do become complacent to some degree when diving is "the same old, same old." It's natural. There was a recent article in a cave diving magazine discussing the complacency that comes from "home caves."
For well-trained, experienced and fit divers, diving - even solo diving - is a fairly safe activity. For such divers, dangers faced in the water are usually environmental. Having a buddy doesn't necessarily mean that one is safer. If facing an environmental threat when diving with a weaker buddy or newer diver, a diver may not only need to manage the environmental threat, but may also be placed in the position to be a rescuer.
An experienced diver facing an aggressive shark will know to stay cool, stay on the bottom and move slowly and deliberately. An inexperienced buddy could swim erratically and escalate the shark's aggression.
An experienced diver caught in a current will know to stay calm and determine the type of current and whether to swim to get out or relax and signal for assistance from passing boats. A weaker buddy may become a liability.
Entry, exit and the point of lowest gas at maximum depth are the most dangerous times during normal dives for both solo and buddy divers. Entering without all of one's gear or with air turned off can be problematic. Losing gas at max depth and max distance from safety poses significant risk if errors were made in reserve gas calculations. Exiting the water after a long dive, even if you aren't tired, can find you a bit ungainly and unbalanced when returning to being a land mammal. After a long day's freediving, I often have trouble standing up without waiting for my knee to compensate for the three torn ligaments I currently have - ACL, MCL & PCL!
Boat ladders, as have been mentioned, need the greatest respect. On a dead calm night off the coast of Florida, there wasn't a ripple. I was hanging on the swim platform talking to the boat captain I crewed for as my night diving student climbed the ladder. A small wave picked the boat up and slammed it into my chest stripping my back up lights from my harness and breaking the bolt snap of my long hose. I dove to recover my lights. I was lucky not to have been hurt seriously.
Ultimately, risk is a personal thing. Some of us want to live to a ripe old age. Some of us don't. Some people are in excellent health and look forward to continued joys as they age. Others are facing health issues as they age that an accidental death would make merciful. Some people are defined by how they live. Others are defined by how they die. Society tends to arrogantly thumb their noses at those who place themselves at higher levels of risk and defines such people as somehow being less evolved, mature, or intelligent. Solo divers are often thrown into such a category. However, I personally respect those who know what they risk, why they risk it and own their consequences.
While hiking the other day, I was thinking about the ways I'd like to die and at what point I'd be okay with checking out. I was thinking about how safety-obsessed we've become as a society. Too many cubicle-dwellers, bean counters, and attorneys and not enough Point Breakers. By the way, John Philbin (Mr. Carter) from the film is a great surf coach if anyone wants to surf or improve their surfing. I realized that there were never any surf conditions from RI to Va Beach that I didn't try to paddle out into no matter how big, nasty or closeout. A lot of surfing I did alone at night. Good times!
Philosophically, I think the most dangerous aspect of a solo dive is the fact that few people understand the human spirit that drives divers to dive alone.