coming from someone who has thousands of scuba dives in the keys, and hundreds of hours snorkeling...
there are better reefs than Pennekamp...
Looe Key sanctuary off Big Pine (look up Underseas Dive Center or Strike Zone) is the nicest reef that is regularly visited.
much of what they take you to on the cattle boats in key largo is pretty busted up from traffic.
depending on when you come down, look me up...boats in the water and i have plenty of experience. located right in the middle of the keys in marathon.
if the weather is nice enough i'll take my speargun and shoot you guys some fresh fish for dinner.
places to call up in key west for trips would be Subtropic Dive Center or a web search for Keys Scuba Shack (mike is the owner and is a great guy)
good places off the shore to snorkel are hard to find. not alot of structure close to shore other than small little stuff.
Sombrero Beach in marathon has some rocks and culvers in the water that are pretty cool. the beach is beautiful and the "structure" is within a swim zone so you don't even need a dive flag.
Just south of the southern Bahia Honda bridge is a rock quarry on the gulf side, has a small access road to the highway. its pretty cool. has some shallow stuff and goes as deep as about 30ft within 10ft of shore. even has a boat hull in the bottom (hard to find) and an upside down truck (circa 1950s) burried up to the doors.
keep in touch...