Sort of like shaken baby syndrome taken to the tenth power and delivered in one second or less. That’s a risk nobody would have known about then, especially how non-compressible water transmits shocks bar better than compressible air.
I once tossed an M-80 into an empty metal pail. When it went off, it made a dent in the bottom.
Then I filled the pail with water and repeated the experiment. The second M-80 ripped the pail at its seam and flattened out the sides.