We have returned from our trip. We recommend taking taxi to/from airport to cruise port as the price is fixed … $19 for 5 pp, $2/add pp, $1/each luggage. Some taxi drivers may want to charge more perhaps include good tip.
Taking taxi within Old San Juan can be expensive compares with the airport rate. One can walk or take the free trolley. It is a small city, narrow streets, traffic, steep slopes, and difficult to locate a parking space.