1. To have a visibly exaggerated idea of ones own skills, talents, or abilities.
2. To exhibit a grotesque amount of undue and unfounded pride for a skill or ability which is actually mediocre.
Excellent examples of people who are really bad showboaters would be that 'Lord of the Dance' blockhead Michael Flatley, the ludicrously posturing Sarah Brightman and her bloated ego, and the Queen of the Showboaters, that swirling black hole of talentlessness herself - Celine Dion.
"This was our one chance to get rid of tha showboating Canadian wench!" - Peter Griffin, Family Guy