Each city and each beach is different.
I've only ever gone off just north of Sunrise Boulevard in Fort Lauderdale and that is fine. I've also seen friends go off John U. Lloyd State Park in Dania, but it was too rough that day for me to launch. I also know that you can launch "north of the pine tree" off Dania Beach, that would be north of the pier and it is a bit of a schlep.
I've heard that friends also go off Pompano Beach but I don't have specifics.
And remember that you can launch in many parts of the keys. You probably won't paddle 4 miles out to the reefs, but you can still get to some fine areas to snorkel, fish and catch bugs.