From past experience you do not want to drive in Miami, there will be plenty of taxi's and shuttles at the airport to get you where you are going.
If you think they are asking to much for the fare ask to see the price, they all have one and have to go by it. In most cases the same driver will pick you up at the hotel the next morning and take you to the pier. Ours took us to the hotel, the next morning to the pier and the next week back to the airport. You can adjust the price to the pier.