I'm stupified on why you would need to reach your tank valve to turn it on.
First, it should be part of your buddy check.
Second, you should have a bit of air in your BCD before getting in the water.
Third, if your air isn't on, and you get in the water, you should still be able to fin towards the surface if you aren't ridiculously overweighted.
Fourth, you can still orally inflate your BCD until your buddy can help you turn your valve on.
All this talk on it being absolutely necessary to be able to turn off your valve seems odd. Note that I'm talking singles here, and not doubles.
- ChillyWaters