I'd been diving solo in 20-30ft of water in a lake, since I can easily freedive my drysuit to 40. I started reading about solo diving redundant systems, and decided more air and an extra regulator on a twin manifold I had laying around would be a good way to go. It doesn't drive like an SUV as much as it feels like pushing one. A standard fin and kick at a standard pace will move you along at about 6 inches per kick (with no current).
The real beauty is that I'm now exploring places that I'd never seen before, well beyond the horizontal range any other diver would have. Its turning all the old dive spots into new adventures.
I don't have enough experience to be anything close to an expert, but I do have a couple of pieces of advice. Have at least 65lbs of lift for a BC. If your going to be in the current, it better be a drift dive, a crawl dive (extra, extra weight), or aided by a scooter. I've got a scooter in mind that is fast enough to fight the current, but without overdriving the headlights in the murky Puget Sound, make sure the batteries handle longer dives, and you can stay warm for two hours. Chill'n leads to cramp'n, especially when your push'n twins (sounds like childbirth - sorry
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Most divers that die, drown from running out of air. Its nice to have a nearly 1.5 to two hour dive, and return with nearly 1000 PSI. One of the dive shops even filled me up for one tank charge fee, since I had so much left over!
Really think about getting a lot of buoyancy. I'm thinking 85-90lbs of lift on a wing would be better. I want to be able to use the wing like an inner tube and have a chance of swimming back to shore against a light current. Diss the equipment and swim, if your life depends on making back to a boat of shore. A current is going to win against the twin.