If you're getting swamped swimming on your back and aren't overweighted a 7" plastic tube isnt going to help.
I am not sure why you use the snorkel swimming on you back. Isn't the snorkel used when swimming with face down in the water?
Ive seen the cause incidents at and below the surface. We've banned the wearing of them from anyone using our club boats for safety reasons. If someone really wants one it must be stowed in a pocket.
What incidents and what type of diving environment are you talking about? I'd think that this is a drastic action for O/W recreational diving, isn't it?