My biggest disappointment with Treviso airport was the discovery that there are no water taxis. A friend had told me that the water taxi from the airport into Venice itself was the only way to arrive, because, aside from being one of the romantic wonders of the world, it lets you encounter the city from the water, the way it was designed to be seen. It was only after purchasing the flight that I learnt this option is only available from Venice's other airport, Maro Polo.
The Ryanair ticket from Stanstead to Treviso was a mistake all around. It seemed like a money saver at the time. But add a checked bag (20 pounds) and the train ticket to Stanstead (28 pounds each), and the savings were pretty meager. And the journey took forever. On top of the hour-long train/bus connections at either end, on the return leg we had to stand in a sequence of lines for five hours, because of crowding at the airport. Treviso and Stanstead have poor facilities for handling large numbers of passengers. Avoid Ryanair's Venice/London flight if you can.
After the rude Ryanair experience, my friend's words about the water taxi were a challenge. I was determined we would try it, at least once. So we took the ATVO bus from Treviso to Mestre (50 minutes, 6 euros each), then the ATVO bus from Mestre to Marco Polo airport (20 minutes, 3 euros each). It was actually very easy, with hardly any waiting. The water taxi from Marco Polo to our hotel was around 100 euros, more than the flight. It was thrilling from the outset. A journey I will never forget.