Lorine, coming in from Marco Polo airport, there are two bus lines with frequent runs from the airport to the Piazzale Roma. The Piazzale Roma is a major vaporetto stop as well as the main stop for the cruise terminals. The bus ride will cost a few Euro and will take 20-30 minutes. You could take a regular taxi but it will cost more and it will take you to the same place. The bus is better.
Once at the Piazzale Roma, there are two main modes of transportation on the canals in Venice. The primary public transportation is a water BUS (vaporetto) and not a TAXI. The second is a water TAXI which is private and not public. On the water taxi, you will pay about 100Euro ($140) for up to 5-6 people to ride. Very expensive (the Rialto Hotel website says that as well). The water bus costs 6.50 Euro each way unless you have a tourist pass of some type.
Once at the Piazzale Roma it is very easy to get to your hotel on the vaporetto. Take either the No 1 or No 2 vaporetto going to LIDO. The No 1 stops and every stop. It's the local. It will take a bit longer. The No 2 is an express and will get you there a several minutes faster. Either is fine. Once at the Rialto stop, you will need to cross the Rialto bridge. You will bumpty-bump up and down the stairs on this bridge so packing light will help. To return to the cruise terminal, just reverse and take the vaporetto back to the Piazzale Roma. The cruise terminal is a 15 minute walk. There may be shuttles there - I've never walked to the cruise terminal.
