In the Venice airport (where you get your bags) there is a machine to buy tickets for the bus to take you to the main parking lot in Venice. They're a few euro (machine takes credit cards too).
You can then buy a vaparetto pass and take the vaparetto to your B&B. The B&B should give you instructions to walk from the nearest vaparetto stop without you having to ask.
Do not take a water taxi - extremely expensive! almost 7 years ago
Bus is the cheapest way but the nicest way to arrive in Venice is to take an "Alilaguna" vaporetto. Takes 45', very efficient and comfortable. 15 euros one way, cheaper buying online, cheaper if you buy your return ticket too. Check the web site www.alilaguna.it. You can go all the way to San Marco square stop. over 6 years ago