Actually - This reminds me of a "Funny Ha-Ha" that is some years old.
Now these submersible lift ships are fine, and will go up and down with the regularity of an express lift, providing of course that the hull what goes under the water maintains its watertight integrity.
Some 20 years ago a Semi-Submersible (Wijsmuller BV's "Super Servant Two") was delivering a drilling rig to the mouth of the Congo River.
No problems with the voyage and the ship prepares to submerge to allow the rig to float off.
Down goes the ship, the order 'cease flooding' is given, but the ship continues to sink. Crew etc hastily appear on the superstructure and take to the liferafts. And with a "Glugging" noise the Semi-Submersible is now fully submersed.
Yup - You guessed it, some BF had left a hatch open,. normally above water, but when she submerged this hatch went below the Water line.