I am afraid that this really isn't true. When a tank fails, it usually splits and lets all the air out at once. If it is being dry filled at the time, the air can escape in every direction equally and looses it's pressure everywhere. However, once you put a tank into a water bath, that failure will have the expanding air pushing against water which is not compressible. So, instead of air rapidly expanding into the surroundings, it is rapidly displacing the water. The water has nowhere to go but up and that is usually not enough to soften the pressure. So the water also expands to the sides causing a drastic failure of the walls of the container. Most dive shops that fill in water use standard horse troughs and those are usually galvanized or heavy plastic. That means some gnarly shrapnel is flying along with the water.
Just my $.02
Your explanation is defying the basic laws of physics. When the air is released from the splitting tank, the force is going to go in the direction of least resistance and since water is denser than air, the majority of the force will be going up (the spot of least resistance). I'm not saying it will all go up, but the greater amount of it will go up. This will leave the rest of the force that does go in other directions less than if it was not in the tub. Also, the water bath will actually absorb some of it and the majority of it will be going up. What's left, will of course go in all directions. The container will also absorb some of the force, again reducing the damage done by the explosion. Tubs will not throw shrapnel, but in all likelihood will split, leaving a rip or rips in the tank. Bottom line, if the operator is in the room when a tank does go being filled, the operator in most cases will not know what hit him or her; the percussion alone will knock them out, if not do them in (and a tank of water will again reduce this percussion, absorbing some of it). This is not opinion but nothing more than a basic understanding of the laws of the physics.