If you look at a picture of the Disney Dream, for example, you will see that below the outside promenade, there are 2 decks with relatively big port holes, the bigger ones on deck 3 are from public areas, the smaller ones one deck below are guest cabins.
Then, another deck below, deck 1m there are pretty small portholes, they are for crew areas like mess hall and cabins for medium rank crew, then a lot of inside cabins for regular crew is on that deck as well. So this deck is still above water level. The next 2 decks, usually called decks A and B are probably mostly storage rooms, preparation kitchens and technical areas, but some cabins might be here as well.Most crew have 2 bed cabins and they usually are even smaller than the smalles guest cabin.
I once got a tour of crew areas on another ship and it was really obvisous, how close the cabin doors were on the crew decks, also, the corridors are much narrower. However, most modern cruise ships usually have a big central corridor on which you can walk pretty much the full length of the ship, the ship crew often gives them a funny name like Route 66, I-4 or so.
They have very long work hours, so they don't get much rest at all. Also, they work 7 days a week for several months. For example the dining room waiters, they have to be ready before breakfast service starts, as not all servers are needed in the dining room at breakfast, others will have to help out at the buffet, which often opens earlier than the dining room, so they might have to get up at 6 or before. Then they might geht short breaks between meals, but probably not more than an hour or 2. Once dinner is done, they have to clean up the dining room, get it ready for breakfast again and then might have a meeting where the next days meal are discussed. They might get out of the dining room by 11 PM or midnight.
That's a pretty hard work schedule.