You know what, I’m not so sure that this comment is correct, either. A little bit of online research indicates that while fixed ballast has been used in the past for surface warships, it’s usually been an add-on fix to improve stability. Also, I can’t find any modern warships that indicate that fixed ballast is present, only old designs from WWII or earlier, and many of these were corrective measures.
The ballast mentioned for surface warships actually seems to be variable water ballast, like Walther Ego indicated.
I’m going to restrict my comments to submarines, which is the only class of vessel I actually served on. I guess my half-remembered “Introduction to Naval Design” class from 20+ years ago shouldn’t be used to make blanket statements about other types of vessels.