Where in the Keys will you be staying?
If you haven't decided and snorkeling/kayaking are high priorities I'd recommend the Middle Keys, probably Marathon on Vyca Key just NE of the Seven Mile Bridge.
Will you be prepared with equipment to get your collected items home safely?
I use a big bucket and a battery powered bubbler with a weighted airstone for use at the beach or out in the boat. I use a medium size cooler and a plug in bubbler back in the motel room. A heater isn't a bad idea if you keep the cooler in your room. A/C in the room and the bubbler blowing cool air through it can get the water quite chilly over night. I also do water changes at least every other day, every day if I can.
Have you read through the thread here:
http://www.reefcentral.com/forums/showthread.php?t=2511350
There are a variety of places described that could work for you. The Horseshoe off Spanish Harbor Key (or the other side of the island) are both just walk in off the shore.
Little Money Key is 1/4 mile off shore, Money Key is 1 mile off shore and Molasses Key is 2 miles off shore. All 3 are along the Seven Mile Bridge and in quite shallow water. Just be aware of tides with a kayak. Like Sam said above, just turn over a rock or two and you'll have stars, sea cucumbers, emerald crabs, rock flower anemones, snails and hermit crabs. There are a lot of other things as well, but it's very unlikely you'll want them in your aquarium at home! Divers gloves (or some other kind of protective glove like cheap mechanic's gloves) can be a big asset. Some of the rocks are sharp and have things like fire worms hiding under them. BTW, we lift rocks up, look under and set them back down the same way they were. Flipping them over exposes sponges and other animals to sun and predators that they can't survive. Do your part and be a good citizen for everybody else who wants to snorkel as well as for all the critters in the water. Life on the reef is tough enough.
As you can guess, some of these have come home to my tanks (pics 1, 4 & 6) and some (pics 2, 3, & 5) are still in the water back at the Keys! I hope have great weather. BTW, the Horseshoe is quite 'protected' from 80% of the winds in the Keys and with the large arms that stick out, there is always some very calm water to be found somewhere around the outside. Inside is cool/cold as it is 40' deep in spots. Not much to see in there except along the edges out near the opening away from shore.
