a practical joke played on greenhorns aboard ship. A tub full of water is placed between two stools, and the whole being covered with a green cloth, a sailor sits on each stool, to keep the cloth tight. The two sailors represent Neptune and Amphitrite, and the greenhorn, as ambassador, is introduced to their majesties. He is given the seat of honour between them; but no sooner does he take his seat than the two sailors rise, and the greenhorn falls into the tub, amidst the laughter of the whole crew.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Ambassador from Infoplease: