Mot of the historic areas are part of the US National Park Service and are off limits to detectors. There are a lot of beaches in the hotel districts of Condado and Isla Verde that are easily accessible. Some ave less suef than others but there is plenty of sand to search.
The area around Old San Juan is generally developed with only one beach (the old Navy Officer's Beach which is now public). You can hunt at most every public beach and "Balnario", which are run by the government, Most of the balnarios require a small fee to get in, maybe $3/car. If you have a map of PR, you will see that there are quite a few balnarios just outside the metro area and good beaches all over. Bring your best defensive driving skills and a good navigator. Try Luquillo or the 7 Seas beach if you are on the east side. Cangrejos beach is near the Bacardi factory near San Juan and either place is worth a visit.
Water will be about 76 to 78 degrees and the daytime temps in the mid 80's.
Let me know if I can answer any other questions.
