I had never seen 'showboat' used as a verb until a story yesterday about a footballer 'showboating' when taking a penalty, but I am clearly behind the times, as it appears on this children's page (same story). Admittedly, it appears only in the report - none of the children uses the verb in their comments (which, incidentally, are a lot better spelled and more grammatical than most internet forum comments).
To showboat means to show off or behave or perform ostentatiously (OED definition). The OED describes it as US slang, and says it dates back to the 1950s. As a noun, a showboat is a show-off, as well as, of course, a river steamer with theatrical entertainment. That sense goes back to the mid 19th century.