Earlier quoted context omitted.
You're literally the only person I've ever seen make this claim. Why do we get so many stories of sleep deprived sailors if ships are over-crewed?
In my time on a ship we were always undermanned. All the development I've seen in new systems to replace those I worked on (arresting gear and catapults) focus heavily on reducing required manning. We never did busy work or work that was unnecessary when we were under way and flying aircraft. In port - yeah sometimes. Most current sailors I talk to are overworked, training gets gundecked, following safety procedures…
It's not so much that we were doing "unnecessary" work, but rather that many tasks which required a dedicated person were so trivial that any reasonably competent sailor could easily handle two or three of them at once, or the task could be even more reliably automated.
One quick example is the fathometer operator: When we were sailing in shallow or uncharted waters, a sailor had to spend hours doing absolutely nothing but watching the depth gauge so he could give a verbal warning if we were in danger of running aground. This is not a job that requires a person's full attention even in the most challenging undersea terrain.
To some extent it's necessary to break jobs down into simple repetitive tasks so that they can reliably be done in a high-stress environment, but the navy really took it to an extreme. The end result was the overwork and fatigue that others are referring to.