Photos of the sunken WW2 German battleship Bismark show all 4 main gun turrets open & the actual turrets & guns missing. Why & how?

They're down there, just not with the rest of it.
http://www.bismarck-class.dk/bismarck/wreck/bismarck_wreck_3.html

It capsized and the stern broke away and the gun terrets fell out before it righted itself. It actually fell on the side of an extinct volcano.
http://www.hmshood.com/hoodtoday/2001expedition/bismarck/encrypt.htm

The Bismark turned upside down (called going Turtle) before sinking. The main battle gun turrets weighed so much, they fell out of the deck structure and sank before the hull did. The turrets are lying upside down near the hull.

The turrets were designed to sit on the deck over an opening, they were not designed to hang upside down from the deck. To better understand the concept, set a small bucket to represent the gun turret inside a bigger bucket that represents the ship. The small bucket will stay in place and rotate inside the larger bucket but if you turn it upside down, the small bucket will fall out.

The guns of battleships are such massive bits of kit that they rely largely upon their upon mass to keep them in position. Don't forget that the Bismark took one hell of a hammering before finally sinking.

The pilot that got it put the bomb straight down the stack. Pushed everything outwards.