Two of the hotels I have stayed in previously (both on non cruise visits to Venice and pre or post cruise) are the Best Western Olimpia and the Santa Chiara. Both 3 star but clean and comfortable. The Santa Chiara is right on the Grand Canal and the Olimpia just a few minutes away over the Tre Ponte Bridge. Both hotels are adjacent to the Piazzale Roma bus terminus and a 10 minute walk from the train station.
Depending upon your taste, their is a beautiful Five Star Hotel called The Santa Monica right on the Grand Canal, most of the Gondola's are moored up outside and the restaurant and views are wonderful. You can get a water taxi from there around to the port which is less than five minutes. The Hotel is just five minutes walk from St Marks Square. Worth a visit.
We took the same cruise in September 2007, our first cruise, we had a brilliant time. We flew out the day before and stayed in a Holiday Inn in Mestre, Marghera, just on the outskirts, but within easy distance of Piazza Roma - they provide a courtesy bus for a small charge - we took a taxi to the port the next day which we cant quite recall the cost but thought was quite reasonable at the time (booked through the hotel).we managed to get to the hotel by local bus from the airport. Holiday Inns are pretty standard wherever they are, quite a lot of Americans who were also on this cruise were also staying at this hotel through Royal Caribbean.
We are sailing from Venice on 15th Aug and like you need to spend I night before. We have booked into Casa Sant'Andrea a 2 star hotel we found on Easyjet site. Its only £77 for 1 night and its located near the Piazza Roma where buses from airport come into and free shuttle buses to port go from. Can't vouch for hotel but it was cheapest we could find!!