The BEST way is to take a taxi. It's about $20 + tip.
The CHEAPEST way is to take the local public bus to the Justice Complex for $0.50 pp then walk across the street to the tourist bus to the port for $1 pp. Total cost for 2 people = $3.
I was also told that if you are renting a car from some of the big rental companies or are one of their premier members, their shuttles sometimes will drive you to the port for free. I do not know if this is true.