If you are doing a beach snorkel to collect I'd recommend The Horseshoe and the other side of the same island. You can walk up the hill and back down the other side of the road or if it's slack tide you can swim around under the bridge. But if the tide is going out or coming in, that swim can be very difficult. On both sides you'll have a wide variety of different environments available. It would be easy to spend more than a day just exploring these two sites that are only 100 feet apart.
The Horseshoe on Spanish Harbor Key by
Ron Lindensmith, on Flickr
If you have the kayak and the winds are reasonable, put in at the boat ramp at the southwest end of the Seven Mile Bridge and paddle over to Little Money Key (about 1/4 mile) to the northeast. The distance is short, but you are crossing a tidal flow channel and it can be a challenge to cross safely when the tide is coming in or going out in a kayak (we do it in a Zodiac with a 30hp outboard). The north side of the island, from the dock on the east all the way around to the northwest is shallow. We like the area very near shore (barely 1' deep) near the dock and around to the north. Much of the bottom is covered in broken finger coral (stony and illegal if alive) with lots of sponges and green Little Money zoas. Pick up a piece of loose coral and the odds are you will find a Mithrax crab. There have been brown palys around the area under the dock.
Little Money Key by
Ron Lindensmith, on Flickr
Both places offer lots of potential to max out your legal limit of 20 animals. Remember, only 5 polyps of zoas per licensed person per day (just a basic Florida Saltwater fishing license). Florida FWC is a regular visitor to both sites and my collection has been inspected more times than I can remember. We are around mini lobster season (small season not small lobsters) and FWC is really out in force at this time of year!
One last request. Please share your experience with us. Pictures are great, but even just a short paragraph detailing what you found and what the conditions were like. We have not been to Little Money Key since Hurricane Irma and I'd love to hear a report on conditions under the water!
Good luck and thanks.