There is no diving in Merritt Island - that is worth it. You must go 15 - 25 miles off shore and it is not very good and no scheduled dive boats make the run. This is home to me so that part sucks. First real good diving in the ocean is in Jupiter and then anything south. As others have said, you also have Cave country to the north with places like Devil's Den and Blue Grotto. Keeping close, I would go to Jupiter. Jupiter has some nice wrecks and lots of big fish. I like Stuart Scuba and Sl8r for that area. If you are into shark feeding and big apex sharks, Emerald Charters out of Jupiter is also good. There are a bunch of wrecks further south in Pompano Beach. I consider this the wreck capital of Florida. There I use South Florida Diving Headquarters and Pompano Dive Center
Enjoy it!