We stayed in the Isla Verde area pre-cruise in April and walked over to the beach for the afternoon. It is one of the better beaches in the city area and was perfectly safe. Some of the big hotels are there and it is close to the airport. You can take a taxi from the piers maybe 20 min. or so depending on traffic. It would be good for a few hours. I have heard that Luquillo beach is beautiful, but it is quite a long ride from the city and think that taxis would be quite expensive. It is over on the east coast, not far from the rain forest, but if you are only in San Juan for the day, might be too difficult to get to with the traffic and needing to get back to the ship on time.
You are very welcome! Aren't these boards great? I have gotten so much valuable information from them so I like to contribute when I can. I live north of Boston near the border of N.H., about 25 miles. God bless our Red Sox! Hope your cruise is great. What ship are you on? Been on 3 out of San Juan, all were great.