The Coastal vessels (Clyde Line, Ward Line, Mallory Line, Merchants and Miner's Line) were two, and sometimes three, class vessels but the majority of passengers were carried in first- Using Ward Line's Havana of 1907 as an example: 210 First; 46 Second; 24 steerage. Using Clyde-Mallory's Cherokee (1925): 412 first, 25 second.
Second class on these vessels tended to disappear entirely on cruises, southbound voyages, and peak season trips when the cabins could just as easily be sold as bottom rung first class accomodations at slightly higher than second class rates. As late as 1930 (Morro Castle and Oriente) a nominal second class section of the ship was designated (430 first, 100 tourist) but from various scrapbooks I've collected I've learned that the passengers who booked into the supposed tourist class cabins had run of the ship access and ate in the first class dining room.
>but I think that on Queen Mary 2 some facilities are not available to all passengers.
Queens Grill has access to a small private deck aft, which has nothing to recommend it other than a sign that says "Queens Grill passengers only." It is actually inferior to the crew sun deck which surrounds the funnel. Queens Grill and Princess Grill share a lounge which is nice, in a subdued way. Both grill classes have their own dining rooms where one can order off menu: however; they are placed where most ships place their deck buffets, aft on the boat deck, and offer restricted sea views.