If you used the cruiselines transfers, you would still have to go all the way back to the airport to get it...The port is only 20-30 minutes from the airport, much easier, faster and cheaper to get a cab...
I would suggest finding a "taxi companion" while waiting in line at the airport. A lot of the passengers usually go to the same touristic areas,which are basically limited to 4:
Old San Juan
Just don't be shyand dare to ask people around you!