I haven't done the walk to a cruise ship, but I did it to a ferry to Sardegna, and it's SO FAR.....I mean I bet it's at least a half mile, probably more depending on where the ship is.....
If you're on the freccia trains from Florence to Rome, there is space between the backs of the seats of the pods of 4 that face each other (if there are two pods, then there is dead space in between that will probably fit 1 suitcase, MAYBE 2) the backpacks can go in the overheads, but if those dead spaces are taken (more likely if the train doesn't originate in Florence), then your luggage will go at the end of the train car in the luggage storage places. On the train from Roma to Civitavecchia, it is most likely a regionale train, you will just have to make do on where to put luggage, do NOT put it on a seat and take up space that a person could sit (and absolutely do NOT put your feet on seats), there might be space in your row or pods of facing seats, but most likely it will be in the aisle....most likely there will be overhead space for the backpacks....