San Juan has tourist taxis at the airport and at the port that are run and assigned by the PR Tourism agency. The taxis have a flat rates by zone, so you know exactly the rate when you get in the taxi and you won't get ripped off. Take the taxi, it'll be cheaper.
Majesty of the Seas - 10/03 & 02/07 & 11/08
Enchantment of the Seas - 10/04 & 10/11
Mariner of the Seas - 10/05
Vision of the Seas - 10/06 & 09/07
Carnival Liberty - 10/07
Adventure of the Seas - 9/08
Ruby Princess - 12/09
Voyager of the Seas - 9/10
Carnival Spirit - 11/10
Oasis of the Seas - 9/13
We spent a miserable hour and a half waiting in the hot sun trying to get to our transfer, with mobs gathered to get on, no line control, and nobody from the cruise line supervising. Hopefully your experience will be better.
Another time we went to the taxi rank, negotiated with the dispatcher for an hour tour of the city and then to the airport, and the driver lost all his English and took us right to the airport and then demanded the negotiated fee. Again hopefully your experience will be better.
Next time we go to San Juan we will definitely book a private limo transfer.