WordNet (r) 3.0 (2006):

showboat
    n 1: a river steamboat on which theatrical performances could be
         given (especially on the Mississippi River)

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "showboat":
   Elizabethan theater, Globe Theatre, Greek theater, amphitheater,
   arena theater, auditorium, cabaret, circle theater, club,
   concert hall, exhibitionist, flaunter, grandstand player,
   grandstander, hall, hot dog, house, little theater, music hall,
   night spot, nightclub, opera, opera house, outdoor theater,
   playhouse, show-off, theater, theater-in-the-round, theatron