My family has lived down there for generations. Those islands can NOT afford to have every other tourist taking conchs/seahorses/crabs/starfish/sand dollars/sponges/etc. The area, despite what some may say, has been declining my entire life because of people taking what they want. They are much more strict than they used to be, but still cannot possibly catch every person attempting to take sea-life from the water or mudflats.
Please, look all you want, and take a couple beautiful shells home with you, but don't take the wildlife.
There are a few areas, adjacent to the mainland, and not the islands, where you can find nassarius snails, hermit crabs, horseshoe crabs, etc. Most notably "Bunch Beach" on your way out to sanibel. I see this area listed as "preserve" in some places, and I myself havent been there in several years. I would look into that if you want snails and crabs, but I am still unsure as to the legality.
Please, if you are going to take any wildlide, make sure it is not from a protected area. Most of the best stuff, unfortunately, is located in a protected area. You will see a few signs while on the beach, and they do have wildlife officers on 4-wheelers preventing people from taking live animals.
I'd recommend doing some fishing, which has bounced back tremendously over the last 13 years since the net ban. There is great fishing for (when in season): grouper (offshore), redfish, tarpon, snook, jack, snapper, and shark (lemon, black-tip,etc).
Sorry for the rant...have a great time, do some fishing, enjoy the view, take a few shells, and just forget about taking livestock.
-Ed