Crabs are great bait for many species in Florida waters. We have Blue Crabs, Calico Crabs(that many people call Pass Crabs), Fiddler Crabs, and Sand Fleas(I think they are crabs, if not, they should be). Of course, there are many more varieties of crabs in Florida(I've had a lot of old crabs on the boat) but these are the most commonly used for bait.
Blue crabs are used inshore primarily for drum and permit either whole or broken in half. Offshore they are used for almost anything. Just about every species in the Gulf will eat a crab but I use them primarily for cobia, hog snapper, grouper, tarpon, and if I'm out of other bait, whatever bites is definitely what I was fishing for. To hook a live Blue Crab, most frequently the hook is run up through the bottom and out the top shell at the point of the shell on either side or in one of the back leg openings and out the top of the shell. This allows the crab to move more or less naturally and stay alive for a while on the hook. If you are fishing on the bottom, you need to remember to move your line frequently because the crab, while alive, will try to hide under anything on the bottom in order not to be seen by predators. You can also cut a large crab in half if you think you need a smaller bait and hook it in the same place. You can also do the same with frozen crabs if that is all you can get.
Pass Crabs are almost identical to Blue Crabs in shape, but they only grow to approximately four to five inches in width. You can catch Pass Crabs in south Florida during the outgoing tide on the surface, usually floating along with grass as it flows toward the Gulf. You simply take a long handled net and dip them out of the water. There are a few bait shops in south Florida that sell them, but they are few and far between. The primary use for pass crabs is for tarpon in the Boca Grande area, but they are just as good , maybe better, than blue crabs for a variety of fish. Hook them in the same way as blue crabs, through the back leg hole and out the top shell.
Fiddler crabs are much smaller than Blues or Pass crabs and are primarily used inshore for pompano, permit, sheepshead and an few other crab eaters. You can catch your own fiddlers or buy them at many bait stores throughout Florida. Again, like the other crabs, hook them through the back leg hole for the best results.
Sand Fleas are about the smallest of the four crabs, usually not larger the a quarter. You can find them live in some bait stores and frozen in many of the stores. They are very simple to catch yourself, if you know where to look. You will need a Sand Flea rake, and the tackle store where you buy the rake can generally give you some information on where to start looking for them. If they can't help you, then you probably should look for another store to spend your money in. They are found on the beach, at the surf line, digging like crazy to disappear before the wave recedes and leaves them high and dry. If you see one or two, then you should dig there, because there are usually many hundreds together in one area. To hook the Sand Flea, come up from the bottom and out the top shell near the back. They don't live very long on the hook, but that doesn't seem to bother most of the fish you are after - they will be eaten very quickly in most cases.