Congratulations on your first cruise. First, don't buy into the frenzy that Carnival is bottom of the barrel. The media has been extremely vicious in it's reporting of Carnival the past few months. Yesterday was another example when all the headlines are saying "Carnival Fascination fails inspection". The media is being shameful in that report by using Carnival as the headline. The facts are, several other ships from other cruise lines had lower inspection scores than the Carnival Fascination.
Don't start your cruise with negative expectations. Go with a positive attitude that you will have a good time.
It sounds like you're already planning to drive in to Charleston a day early. If you're doing a park & cruise package with the hotel you essentially have paid for the parking while you're on the cruise. So why not let them shuttle you to the ship. On the other hand; parking at the ship may cost more, but it is convenient and a time saver when you get off the ship.