Um, auxiliary engines...
backup auxiliary engines...
tertiary backup auxiliary engines...somewhere...?
Okay, I've got it. As far as I can tell, the stern of the bottom level seems unflooded--after all, Silver was able to access it. Let's just say that one of the emergency diesel generators is located around there, is unflooded, and is often used as a handy storage for food.
In the future I will confine myself to exploring the spaces that are definitely unflooded, and will not venture into the Schrodinger engine rooms.
__________________
Vanvikan, Feb. 2009: ordinary human, KIA, night 4
HMS Thanatus, May 2009: ??? human, KIA, night 7