Just a suggestion since you live in Jacksonville, I would do a one way car from Enterprise and drop the car off at their office at Radisson on Biscayne Blvd. Then take a cheap cab ride to the port from there. It would probably be a lot less expensive than paying for parking. then do the reverse when you return or Dollar rental car is even cheaper but their office is near the MIA airport which is quite a bit further from the port but they do offer a free shuttle.
I usually book Dollar through carrentals.com