This is another one of the 5th Dimension's hits that was written and originally performed by Laura Nyro. When she sang it in concert, Nyro would introduce it as "A little drinking song I wrote."
The 5th Dimension sang an abridged version of this song with Frank Sinatra on one of his television specials in the 1960's. Marilyn and Florence looked right at Sinatra and smiled during the line "You're a good lookin' riverboat."
Suggestion credit: Ken - Louisville, KY, for above 2