Venice's Marco Polo airport is about 20-30 minutes, depending on traffic, from downtown via the ACTV express bus, which costs about $9. Private water taxis will collect you from the airport and deposit you in front of your hotel for a fee that fluctuates annually but seems to hover around $170.
The public transportation system in Venice, ACTV, operates along the Grand Canal. It connects passengers to the major points of interest including the Piazzale Roma bus station, S.M. Novella train station, and landmarks like the Academia Bridge, Rialto Bridge, and St. Mark's Square. ACTV also services other lagoon destinations like the Guidecca, Burano, and Murano. Water taxis and gondolas that ferry travelers across the Grand Canal are also available.