I just realized that in San Juan the ship is docking at the Pan American pier, not the San Juan Cruise Port Terminal in Old San Juan. We'll probably be able to see the ship across the bay - so close, and yet so far! We were planning on walking downhill from our hotel in Old San Juan down to the port, but we'll need to take a taxi.
It looks like the Pan Am pier is in the same taxi zone (4) as Old San Juan , so fares should should start around $10, with $1 a bag and possibly a gas surcharge on top.