On trenitalia.com , if you prefer only 1 change at Milano Centrale: from Firenze SM Novella to Como S. Giovanni (or Varenna-Esino). Then walk to the docks and take a ferry to your final destination.
If you don't mind making more transfers, on trenitalia.com enter your final destination in the "to" field. Then click search and see what happens.
I wouldn't completely trust Google Maps to plan my train trips, but You may need a better guide if yours does not list the English versions of Trenitalia.com and Italotreno.it.
Also the most efficient way to get from Lake Como to Rome?
Assuming "most efficient" means fastest, take a train from Como to Milan and then an high speed train from Milan to the station in Rome closest to your final destination. Usually it's either Roma Termini or Roma Trastevere or Fiumicino Aeroporto.