Notwithstanding William Wellington's unquestioned expertise, I have in my possession a shotgun that was immersed a few inches at the muzzles to shoot a barramundi.
The perpetrator was lucky the owner at the time was unarmed!
I was able to salvage a lovely 25-inch barreled double-slugger after the whistle-cocked muzzles were sawn off!
It has since killed many boars and even a full-grown buffalo cow with one shot!

My understanding of obstructions (including pond-water) is that the failure will occur when the charge meets the obstruction. Can't imagine anyone immersing the barrels full-length in the water! Barring forge-welding faults, I suspect a neatly blown chamber like that ML was either a double charge or a double load rammed home. A poorly-rammed load that moved up the barrel an inch or so under first-barrel recoil may also be a possibility?