[andromeduck]

That's just wrong. The the blast from the tank would have to displace water before of the energy of the blast can exit the side or top of the container. That spreads out the energy from the blast and the container would redirect much of that energy upwards both of which reduces the effective radius of the blast.

[A_Person]

I'm no scientist and you may well be right. But I'm skeptical of your statement that "much" of the blast would be directed upwards. If you look at the pictures of tank explosions, they often show the entire room flattened right to floor level - not a big hole in the roof and minor damage everywhere else.

[zamalek]

The shopkeeper is still somewhat wrong though: it's redirecting the energy, not absorbing it.

Regardless, Mark Rober and The Backyard Scientist have a pretty great video demonstrating exactly what is going on here: https://youtu.be/W4DnuQOtA8E