HH, to take the cruise ship case. True they probably do 345 days per year but although large and complex have more space and less electronic complexity.
Refits may take 2-3 weeks but I expect the additional cost of such rapid maintenance is offset by the quicker return to comment service and profit.
Then a crew of over 1,000 can absorb a new personnel more easily. On a typical 8 month contract it means a crew may be completely replaced in 2 years. Greater complexity and smaller crews mean RN crews need to train and learn more complex skills. Also a Naval ship does not work in isolation. You can't afford to send ad hoc crews out on operations as other Navies have discovered.