On your own the boats are less crowded and you will have a better experience. We used Soto's and you can google them. They did a two stop trip for $30.00 per person. They picked us up right by the peir.
I agree with the rest. We've been on a few cruises now and have learned that you can do most things the ship offers on your own for a big savings. The only benefit with booking with the ship is that if your excursion is late coming back the ship will wait for you. They will not wait for you if your out on your own. So plan accordingly.
Cruising.....The only way to travel
Carnival Destiny 2005
Carnival Miracle 2006
Carnival Victory 2007
RCI Liberty of the Seas 2008
RCI Allure of the Seas 2012