I was as surprised as anyone. We've tested several striker fired pistols and the M&P was the first to have such serious issues with the water. We checked and double checked, and this particular gun does not like having water in it. I know some folks will swear up and down this is impossible, or their S&W's have hundreds of thousands of trouble free rounds through them, but what you see in the video is what really happened. <shrug>
Something something... "you did it wrong" something something... "mine wouldn't have done that".
As snug as the M&P striker assembly fits in the striker channel, I'm honestly not surprised with the hydrolock issue. There is one small drain hole and he actual hole for the firing pin is much smaller than on the Glock as well... no where really for water to go in a hurry. I wonder is some modification could be made to work like maritime cups for the M&P?