Banjonyc,
Some advice from someone who was born and raised in Palm Beach County. Just goto any of our beaches and detect, don't do anything stupid, and you will be fine. Some beaches closes at dusk......I still detect. Don't be a nuisance to anyone, don't give anyone a reason to call someone to make issue of it. The reality is, Boca is difficult. Why? Well, because almost all the beaches in Boca require a parking permit. That permit is about $20/day. If you're a resident you can get one cheap (resident of Boca). There is LIMITED non-permit parking, but its metered, and I believe the meters are max of 1 hour. I goto the meters, and I stay for as long as I like, and I have never been ticketed (but YMMV). I typically park at one of the few spots at South Beach Park. Red Reef Park is a nice park too........Boca has some history but much of it is inflated..........I have no doubt there are likely some good stuff in the "Spanish River" and yes some cannons have been found off Boca.........the history is confusing though, just a warning, the location of Boca is not necessarily where the spanish spoke of it, much the same as "Port St. Lucie" is not where the salvage camp was located for the 1715 fleet.
There have been times I have been harassed in Palm Beach. But it was while in the Town of Palm Beach (where Trump lives and many other famous people). I would use Public Access, get on the beach, and walk down below the tide line to very "good" locations. People would see me, call the police, the police would come and literally walk all the way down the damn beach to talk to me and escort me away, and tell me BS like "the beach is closed at dark....." etc. You have to pick your battles. In those situations I just left with them, even though I have a copy of all the Town's ordinances and know they are mistaken. Next time, I just try to be more invisible. That's the thing with our hobby. Legal is not the issue. It's all about perception. You could be perfectly in your right, but still you must obey the orders of LEO's.
Boca is a bit high maintenance. You get a lot of models, wealthy people.......all good for the making of a good day of treasure hunting. But try to not spoil any of their fun. Don't swing the detector in their face, be friendly, don't look like some homeless person who is there to rob them

.........you have to be like an iguana, an adapt to your surroundings. As I said, I hunt Boca all the time, never an issue, great place to hunt, just not friendly because of the damn parking/permit issues, but if you're at a hotel or whatever maybe you have other means to get on to the beach at that point. There are some VERY high end hotels, such as the Boca Raton Resort and Beach club (now known as Waldorf Astoria) is probably very hard to get access into, you can't even dine there unless you are a guest.......but something to check out if you have a way to. Numerous other hotels are good too.
To the South you have Deerfield Beach. A pier area is there, used to be host of some of the major surfing competitions in the 70's. When the tide goes low there is an unbelievable amount of wet exposed, never seen anything like it anywhere else. I have found spanish spikes there there. Also Al Capone bought an island there (Capone Island)........i won't go into it but who wouldn't want to pass a detector on that place (it MAY be illegal to do so, it may be a park that restricts it, I have no idea)........
Delray Beach is huge. Lots of parking, and especially active at the end of Atlantic Blvd and to the South where there is a resort.
Boca/Delray/Deerfield are all nice places to detect. Ft. Lauderdale is a bit more to the South, its massive, and has ALOT more hunters, but its also very busy with people. Every beach has its own great story.