This morning I took a cab from my hotel (15 E) to termini station then the train was 5 euros to the port, taxi was 5pp to the ship (I joined a group I befriended at the taxi stand). There are busses too but not sure if they are only for certain boats.
For timing, I requested the cab at 945, train left at 1042, arrived a little after noon and I was on the boat before 1p. This was with no rushing.