We cruised on Splendour of the Seas out of Venice in September. We booked one night pre-cruise in Hotel Agli Artisti - cheap, clean, quiet, and convenient for the port and railway station. The only downside is that there is no lift, so we ended up carrying all our luggage up to the fourth floor! We could have left it downstairs in the luggage room but as it was not secure, we decided against it. Continental breakfast was included in the price. On the day of the cruise we caught the waterbus from Ferrovia to Piazzale Roma (one stop) from where we caught the free port shuttle bus to the ship.
Venice is NOT the most convenient port to sail from and I'd think twice before doing it again

You would be hard pushed to find a hotel more convenient for both the port and airport on Venice Island than the canalside Santa Chiara in Piazalle Roma. The regular land bus from the airport dropped me at the door of the hotel, it's on a major interchange for the vaporettos (water buses), and the complimetary shuttle land bus to the ship stopped, again right at the door. The downsides: It is not terribly peaceful with all the vaporettos and buses and my single room was miniscule, by far the smallest room I've ever slept in! It's well worth considering though for it's handy location.

Thanks for your replies. Ended up booking Hotel Style in Mogliano Veneto so hopefully this will be ok!

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If you are staying for one night in Venice before your cruise try a gondolar evening serenade it is a little expensive but will leave you with memories you'll never forget.Several gondolars with musicians doing a tour of the canals at night.Peaceful and enchanting.
Enjoy