It was listed on the vehicle list on the game website and in the Bradygames guide but accidently it was replaced with a photo of the Speeder.
According to the Bradygames guide, the Ambassador has an outboard twin engine and has very good acceleration and knot speed.
Another Speeder boat exists in the game files, which is faster than the Speeder and has no in-game name. It is possible that this boat was the Ambassador.
This vehicle is also listed in official website's keybox.xml, meaning the vehicle was originally available in-game.