ChillyWaters:
To the others that have replied to the thread, specifically stating that turning on the valve of a single tank is useful: Why is it so imperative to reach the valve?
I'll give you a local example that was described to me.
Guy and his wife are in a small boat suiting up. Another boat goes by and the wake hits the small boat, and the husband rolls backward off the side into the water and descends since he hadn't yet inflated his BC - his air was off.
The wife sees her husband go into the water, and is a bit miffed because he didn't wait for her. So she continues getting ready and finally goes into the water.
She finds her husband on the bottom - dead.
I actually don't know if he had a weight belt or was in doubles (so had no ditchable weight). However, whatever the detail, being able to reach the valve is not just another tool in the box - I consider it an essential component.
The last thing I do as I'm walking toward the water (or edge of the boat) is to reach behind me and turn on my air. I do this not only as a check that it is on, but more importantly as a check that I can reach it. If I can reach it standing on land, I *know* I can reach it in the water since gravity isn't pulling down as hard when submerged.
Also, to repeat what an earlier poster said, it helps to be horizontal in the water when trying.