Carnival does an excellent 'Behind the Fun' tour which DW and I really enjoyed on the Fasination last year. The Engine Room is really the 'manuvering' area, not the diesels and heavy machinery. you see the 'I-95' lower deck areas, the bridge, manuvering, crew areas and some other areas that escape me now.
As a former MM1/SS with 8 SSBN and 2 surface FFG 'aka' 'target' cruises under my belt, I still find the inner workings of a ship fasinating. Seeing the logistics end of running a cruise ship is mind boggling to say the least. If you ever get a chance to go on one of these type of tours, take it.