It seems you’re not getting much help so I thought I’d jump in! We will be doing that next week…. From Piazzale Roma you can get a bus or train. Bus is easiest, bit it only goes once a day: 5:30. Trains run all day long, but you must transfer. If you’re even a bit adventurous, it should be no problem. If you’re.nervous about it, then go the private car route. We are going a day ahead to Ravenna, so we don’t have to worry about the time. We are also staying at a hotel in .Venice that is close to the train station , same in Ravenna.
check out rome2rio.com for specifics. As opposed to the $300 mentioned in another post to you, plus the water taxi, the train is about $17 to $35 per person.