Water Taxis in Venice - Piazzale Roma

Do you mean a private water taxi or a public water bus, called a vaporetto? Water taxis are quite expensive - likely 70E from Piazzale Roma to your hotel. You can catch either right at the Piazzale Roma.

Auto Europe is nota rental car company. It's a consolidator. If you rent through them you will be renting through some other rental company. Maybe you meant Europcar.
Europcar, like all other companies, are located in a big parking structure at Piazzale Roma. At Piazzale Roma there are vaporetti (water buses) and water taxi (very expensive). There are also regular taxis but those can only take you across the bridge back to the mainland. In Venice you can use only boats or walk. Instead of streets, they have canals and regular cars can't float. If you want to save money take a vaporetto to a location close to your hotel, then walk (vaporetti don't go to the very small canals). For Venice I strongly recommend packing light. No more than one suitcase per person, preferably on wheels, and preferably carry on size. Even with a water taxi you are likely going to have to walk with your luggage, not all hotels are reachable by boat and the canal bridges all have steps.

Thanks Doug and Roberto. I have it zeroed in now!