How long would a drive take from South Beach, Miami to Key Largo?

Question

and would Key Largo offer the best snorkeling opportunities closest to Miami?

Answer 1

1.3 hours.  The "best" is the paramount thing here.  The snorkel boats here all head out about 3 miles to the reef.  You can go as close as Key Biscayne National park (about 1 hour from Miami-SoBe) or do the "stretch" and get into some possibly better (and much better number of choices) in Key Largo.

Atlantis with Cap'n Slate is rather famous, and on the northern edge of key largo.  Keeps the travel time down and good gear/good snorkeling.  When you get back in Buzzard's Roost has excellent food on the Ocean side, right next to Atlantis.  

If you get in for the evening, check out Big Chill or Gilbert's resort for the best sunsets and a relaxed atmosphere at Gilbert's.  Big Chill is more upscale and mid-scale fine dining.  Gilbert's Tiki is great for drinks, sandwiches, and snacks.

If you want a Catamaran trip to the snorkel, check out Quicksilver.  They are A-1 in my book.