My family visited South Miami Beach last week (We stayed at the Holiday Inn). The boardwalk and beach was terrific. There weren't too many large waves, the sand was nice and soft, (not rocky like some of the beaches in the keys we went to). Fair amount of seashells to search for. The sandbar went on for what seemed like forever, and my 4 year old daughter was able to stand out about 50-60 yards from the shore. Water was very clear, and seaweed was minimal. This is definitely one of our families favorite beaches.
Wear plenty of high SPF / waterproof sun tan lotion. I had SBF 50 on and I got burnt. My kids had SPF 85 on and didn't get burned at all.