Msc armonia travel documents

We are going to travel next month(august) from marsaige on board msc armonia.

The itinery of the ship is all inside the schengen zone,i wonder if we can board using our national id(Greek) or we should use only passport.

from the turist agency told us that we can travel with no problem for the specific itinery with our id witout any problem,but some other told me that we strickly need passport,while msc cruises on the web site mention that for itinerys only with in the schengen zone,european passengers can use their national id.

So,lot of complication.

What do you suggeest?

If you are unsure take the passport just in case, you can lock it away in the room safe once onboard .... :)