I don't encourage it...but at the same time I love it. Great fun to be floundering around underwater without having to worry about a buddy.
If you you do it....first of all clock-up a couple of hundreds of dives with a buddy so you become experienced in dealing with different events that may pop-up.
Dive only places that you are familiar with..you don't want to suddenly end up in that surprise current that sucks you into that old net that's got stuck onto that old ship wreck...if you see what I mean.
Always use gear that you are very familiar with.
ALways ask yourself, if I lose any piece of my equipment during the dive or if anything malfunctions, how do I get out of that situation. These things can happen in real life.
- my dive buddy once lost both fins when he went and tried-out a new drysuit, diving with me. The legs on his new suit were longer, his boots weren't very tight, and he didn't use ankle weights. Both boots popped-off when he swam unside down to try and catch a lobster. Visibility was poor, current was strong, gone were the fins... He had to hold on to my shoulder and I finned for the two of us!
- I once had my tank valve tightly entangled in a bunch of old fishing line that was attached to a coral reef. Easy fix when you have a buddy with you. Not so easy when you are alone. If you can't reach the fishing line, there is the option of undoing your tank/BC shoulder straps, turning around, cutting the line and putting your gear back on. Think again...fine in the old days when everyone wore a weight belt...but if you have a weight-integrated BC you might suddenly balloon off to the surface.
Anyway, get the diving experience first with a buddy if you don't have plenty of it already, then get the solo training, and then be fully aware that no amount of training or experience will cover every possible scenario of what could go wrong, so do whatever you can to minimize the risk.
Run what-if scenarios for yourself:
- what if I lose or damage this or that item of gear..it's hard to manage a controlled ascent on a solo dive without a mask
- what if I need to abandon the dive half-way on this particular location... will anyone see me at the surface?
etc...
Make sure someone knows where you are and ensure they know what to do if they suddenly see your emergency buoy floating at the surface.
Practice "things that can go wrong" scenarios, with a buddy present, even if it may sound silly. Ever tried to complete a dive on one fin, or no fins? Try it one day.
SPend a couple of minutes diving without a mask, see how you manage.
Can you manage your dive if your drysuit floods, or if your BC malfunctions?
Can you reach your knife if your leg is entangled in fishing line? Do you carry a spare within easy reach?
In summary, have fun but be prepared to live with some of the constraints.