Water taxi costs a fortune. Dunno exactly how much but it'll make your eyes water. Use the vaporetto, the water-bus. It's possible to walk, of course...
Walk out of train station (Santa Lucia) - get the #1 or #2 vaporetto. Make sure you get on one travelling the right way, as the stops tend to be on the same side of the water. A 24hr travel card is �16pp. Remember to validate it. Water taxis are expensive. Best website http://europeforvisitors.com/venice/