I cruised from Miami to Rome and I didn't find the seas rough. On my Alaskan Cruise we had rougher seas. The weather in early May was wonderful, plus we didn't have to worry about flooding in Venice.
However, if I had to do it again I would cruise Rome to Miami, just to gain the hours on the crossing and have more time on the ship. We lost 5 hours crossing the Altantic Ocean, so going from Rome to Miami you would gain 5 hours.
At the end of the cruise we did take the train from Rome to Venice and stayed in Venice a couple of days (I wish it was longer). If your cruise doesn't stop in Venice my suggestion is to fly into Venice then take the train to Rome. The port is about 1 1/2 hours away from Rome so arrange a good transfer. If I had it to do again I would spend more money and get a private car. Some things are worth the price.