There are many small fish that can be used as live bait in salt water -- Sardines, herrings, several varieties of pinfish, killifish, mutton minnows, chubs and any other fish that fits the size of the bait that you want. Also, remember this: if you try a new bait alive and nothing seems to want to eat it, you can sometimes be very successful by filleting it and using it as a cut chunk or strip bait.
Baitfish are seasonal; therefore I’ll often cast net baits like mullet and menhaden when they’re abundant, then freeze them for use during the winter. Most of the baitfish are gone then, and your fresh-frozen bait will be of better quality than most of the frozen stuff you buy in the tackle stores. To maintain the quality of your soon-to-be frozen bait, do it like this: Soak it overnight in a brine solution of two pounds of salt to a five gallon bucket of sea water, well iced down. Freeze it the next day in Ziplock bags (2-3 pounds of bait per bag), to make it easy to use on future trips. If you don’t care to go to this much trouble, then freeze the bait immediately - it will still be superior to most store-bought frozen bait. Most of the pre-rigged frozen baits are very costly for what you’re getting.
Some of the baitfish description are from Captain Charlie Walker. Visit his website at http://www.floridasaltwater.com