Someone mentioned that the gun turrets on the Bismark weren't "bolted down". Well, no gun turret is bolted down, exactly, because if it was it couldn't rotate! ;-) But I know what they meant. The Bismark's main gun turrets were held in place simply by the force of gravity. They were seated in circular receptacles, and rotating on massive ball bearings. There was no way those extremely heavy armoured turrets were coming out of their seated positions unless the ship capsized, in which case it was a moot point, because if the ship capsizes then you have lost not only the turrets but the entire ship.

This was done on some other capital ships too, not just on the Bismark. If the ship did capsize, then the turrets would fall freely out of their barbettes and sink separately to the bottom.

There is really no practical reason why a battleship-sized gun turret needs to be fastened down to its barbette (the cylindrical column on which the turret rests and rotates). Its own weight is quite sufficient to keep it in place as long as the ship doesn't capsize and sink.