I would recommend purchasing your round trip transfers from the cruise line. It is one less thing to worry about once you depart from home. Also, I have a friend that thought they would just take a taxi and had no problem from the airport to the ship, but after the cruise there was a two hour wait for a taxi to get back to the airport. Remember you are not the only one thinking about this, and there will be probably more than one ship disembarking that day.
Whatever you decide to do, try to make the arrangements before hand, so you don't have to worry about it.
Have a wonderful cruise.