WE have spent 5 days down there in the summer a couple of times. Like many of the above we prefer Marathon at Pirates Cove CG. You need to avoid lobster season which only runs for a few days. But that is July 24th and 25th this year.
We do not find the temps to be oppressive, there is usually a sea breeze to cool you off and Pirates Cove has lots of trees. We are Floridians tho. They have a swimming beach right at the CG and you can snorkel right there. But the best snorkeling is off the tour boats which will run you out to various reefs and wrecks. The wrecks are mainly for SCUBA tho. The tours aren't that pricey. And they usually do two sites each trip. They put their swimmers in the water with you as a safety measure.
We usually do a day trip to Key West, the sunset on the pier is worth seeing. But like most we don't like Key West that much, parking is very difficult and mobs of bar crawlers infest the streets. Somewhat like New Orleans but without the music. Amazingly I was looking at reserving a CG here in Florida this week and Bahia Honda has open slots! Very rare. Indeed almost all of the State Parks had open slots.
One comment on driving to the Keys, we usually get on the Florida Turnpike and follow it to the very end. This avoids the worst of the Miami-Lauderdale horrific traffic. And the road drops you off on A1A a few miles from the start of the Keys bridges. Fuel up right after you get off. Everything costs more in the Keys. Stock up on groceries too before you get down there.