Probably private water taxi, especially with luggage, but its expensive!!
Two most popular ways are public water taxi or private water taxi. The private water taxi will cost about $120 euros (cash only) and will take about 40 minutes. It will drop you right at the hotel's dock. The public water taxi will cost about $20 euros each and will take about an hour to and hour and 15 minutes but is a nice way to see Venice and experience the locals as long as you are not in a hurry and don't have more than a couple of not- to-heavy suitcases. The public water taxi may also have a senior discount. The public water taxi will drop you in San Marco Square which is about a 10-15 minute walk to the hotel. If you are adventurous and like feeling the vibe of the city, the public water taxi is the way to go. If you are tired, have a bunch of luggage and just wat to get to the hotel, then you should spring for a private water taxi. Either way you will love Venice.
The best way and highly recommended is to take a private or shared water taxi. They take you to Westin's docking station.
If you are adventurous like us, we purchased water taxi in advance online. Or upon arrival at Marco Polo airport there is a ticket office where you can purchase a water taxi ticket. The only disadvantage on this is that you have to drag your luggage to where Westin Europa and Regina is and it is not fun if you have heavy luggages.
Email the Westin Europa for directions via water taxi. There is a gondola stop at the Europa. You don't want their version of the local gondola water taxi with quite a few stops - cheapest, but quite long in time. The private taxi will obviously be the most expensive. There is a "middle of the road" option that I have always used. Don't remember the costs. Plan for adequate Euros. The Europa will provide you with all the details of your options. Part of their great service. You will have to make your own way (not that long of a walk) to the gondola departure from the airport.
Motoscafo can be picked up at the airport. It takes you right to the hotel door as it has its own private stop.
Get an answer from The Westin Europa & Regina, Venice staff and past guests.