Getting "robbed" by taxis is really not a major problem at FCO. All the taxis that queue at the airport are properly licensed and would risk their livelyhood if they ripped off customers and had complaints filed with the authorities. It is about a 1 hour drive from FCO to the port and the taxis will charge about 110 - 130 Euros (per taxi). You can negotiate the price before you get in the taxi if this is a concern. There are many transfer companies listed online (just Google "FCO to Civitavecchia transfers") and many of them either list their price on their web site or will quote you a price if you make a request. Generally the transfer companies will cost about the same (and often more) then the taxis but its certainly a legit way to do the transfer. The lowest cost method would be to use the train, but this does mean you need to be able to handle your luggage. The best way to do it on the train is to take the lower cost regional train from FCO to Rome and get off at the Trastevere Station where you can then transfer to the regional train that runs from Rome to Civitavecchia. Once at Civitavecchia its about a 4 block walk to the main port entrance where you can get a free shuttle bus to your ship.