I worked with a man who sank a Sherman tank in the sea in 1944. It was fitted with an amphibious kit and the plan for D-day was to launch them a mile or so from the beaches over the ramp of a tank landing craft.
In a practice session he left the ramp too fast and the floatation kit was filled with water. So down they went (About 30 feet!) one of the crew didn't get out in time and drowned.
So on the big day the whole crew except the driver were stood on the top of the tank, against orders, two of them had hold of his epaulettes in case they went down again so they could haul him out fast. This time he took it gently and the tank floated, went a couple of miles to shore, they'd been launched too early, and fought their way off the beach.
He was a very mild mannered RE teacher and looked like butter wouldn't melt in his mouth.