All of the water taxis are privately owned and operated. They work though dispatch agencies, like Consorzio Motoscafi Venezia. As the other poster indicated, some times, water taxis are subject to tidal conditions. They seem like a reputable outfit, with a 24-hour phone line, and English is the Lingua Franca these days.
https://www.motoscafivenezia.it/en/information.php
As far as rudeness, most don't speak much English, are in the business of getting you quickly and safely to your destination, and obeying the law. They are the captain of the ship and their orders are to be obeyed, like "Sit down!" They don't wan't you falling out of the boat or getting your head taken off by a low bridge, or blocking their view. Since they don't work for tips, they are usually quite direct in their communications. Don't worry about it.