I'm not sure if this will be helpful, as last year we did a Celebrity cruise from "Venice" that actually departed from Ravenna.
In our case, we paid for a private water taxi to take us from our hotel (3 nights pre-cruise) to that Maritime Transit Center (or whatever it's called). There were people movers and other ways to get there, but the water taxi dropped us off on the docks, very close to where the bus departed from. Not sure what MSC offers, but we paid for Celebrity's sponsored transportation to the port and it worked out very well.
When we flew into Venice airport (VCE), we booked a water taxi from the airport to our hotel's private dock. This is a bit more expensive, but really nice and scenic. However, during high tide, water taxis cannot get under some of the bridges in Venice, and so they had to drop us off at the closest public station and we lugged our bags to the hotel, which wasn't a piece of cake due to bridges/stairs/cobblestones and the fact that we don't travel light, but that's what we dealt with.
Again, this may or may not be helpful, since we were departing from "Venice", and on another cruise line, but if it doesn't help you, perhaps it will help someone else.