In the event of a first stage failure you can take the reg off a tank and sip air directly from the valve.
I haven't wanted to screw up a regulator by taking it off a tank underwater but I have swam around with just a tank, sipping from the valve without a problem. Just like drinking from a water fountain.
Tank Rodeo is pretty fun. Pinch a tank between your legs, crack the valve and let it push you around in the water.
I'm still trying to perfect the following scuba prank. Get a bunch of cylinders and put them at the bottom of a pool while the victim is on the surface talking to their students. Have some friends help open the tank valves at the same time. I'm trying to get enough air coming out so the person sinks to the bottom but all that is happening with 3 tanks so far is that the person on the surface gets pushed to the side. We're going to try 6 tanks next time.
Sorry, these aren't really myths.. just a bunch of bored people with access to lots of tanks.