Green’s Dictionary of Slang

1. a self-promoting person; a show-off.

1965		S. Diamond ‘Black Farce, White Lies’ in Dissent 478/2: After all, if he [i.e. Sonny Liston] had gone down for no visible reason, he had aesthetically outwitted Clay, the showboat.
1987		(con. 1967) Bunch & Cole Reckoning for Kings (1999) 20: ‘Don’t be a showboat,’ his coach said. ‘You can kick their butts – but don’t go Hollywood on me.’.
2006		G. Iles Turning Angel 317: Billy Byrd’s a showboat. He cares more about newspaper headlines than he does about enforcing the law.

2. a flashy car.

1972		B. Rodgers Queens’ Vernacular.
1986–7		Maledicta IX 150: The original argot of prostitution includes some words and phrases which have gained wider currency and some which have not […] showboat (flashy car).