Linda, all three boats have rooms with double beds but the categories vary. Your best bet would be to pick up a brochure at a TA or call the DQ office direct. 1-800-543-1949 The room layouts and categories are in that. Their website might have it also - deltaqueen.com.
The Delta Queen's rooms are very small (built in 1927) but are all outside. No verandas as such but with comfortable big rockers to watch the passing scene. Veranda rooms are available on both the others.
The lower Mississippi offers less scenery but some great ports to visit. We enjoyed Oak Alley and Laura Plantation, St. Francisville, and especially Natchez. If you get hooked as quickly as we did, you'll want to visit the upper Mississippi and the Ohio where you will see lovely scenery and truly awesome sunsets.
You can call DQ direct, ask for reservations and book it right with them - they're most helpful. If you feel more comfortable with a TA, most can book it but here's one who specializes in steamboat travel. Phyllis Dale at 1-866-678-7174 or firstname.lastname@example.org
. She's pretty busy so you'd have to be patient.