Wanted:S. Florida Snorkel Tips

I wonder if anyone local in the South Florida and Key West can share some ideas of snorkeling in the area. What are your top pick of snorkeling beaches (locations)? Is Biscayne National Park a good snorkeling place? I'll be staying in in Miami Beach and Key West for 6 days the end of November. Would the water be warm enough for snorkeling and/or swimming?

I'm attracted to the Little Havana neighborhood. Is that a safe place to browse?

Your tips would be greatly appreciated. Thank you very much in advance.