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

A showboat, or show boat, was a form of theatre that travelled along the waterways of the United States, especially along the Mississippi and Ohio rivers. A showboat was basically a barge that resembled a long, flat-roofed house, and in order to move down the river, it was pushed by a small tugboat (misleadingly labeled a towboat) which was attached to it. It would have been impossible to put a steam engine on it, since it would have had to have been placed right in the auditorium.

a person who essentially shows off and tries to draw attention to themselves

Originally a fairground attraction powered by a traction engine the word then began to be used to describe any large well turned out vehicle. The 8 wheeled pantechnicons of Fisher and Renwick were a good example as they travelled between London, Manchester and Glasgow during the dark wartime years.They were always clean and well painted and their striking signwriting attracted attention where ever they went. (Suggested by Oliver Manton with thanks to Frank and Jack Spittle)