Where do crew sleep on a cruise ship?

They are normally birthed below decks in areas near storage and engine room called crew quaters where guests would not be housed

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The crew are birthed in cabins much like the passengers have.

The crews quarters are their home while at sea, so they have access to toilets and showers. Generally speaking crew share rooms, depending on their rank and the cabins tend to be more spartan than the passengers.

Take a look at the link I have added for a picture of a crews cabin on a luxury cruise ship.