Your St Petersburg to Miami via Key West ride is a LONG ride, unless you plan to get to Key West and turn right around and go back. It's about a 6-hour roundtrip from Miami Beach to Key West and back by itself. If you intend to have lunch in Key West, then you're going to have to leave St Petersburg REAL early!
Unless you really want to sleep over in Miami, you could think about grabbing one of the nice campgrounds along the Keys.....there are quite a few places to stay over. Some of them have glass bottom boat trips over the reefs and snorkeling.
Regarding the credit cards, I've never experienced problems as long as I remembered to call the bank first, and have them CONFIRM that they have noted your international travels on your account, to avoid the presumed theft lockdown on the account. Would be wise to have 2 or 3 cards just in case. Visa and Mastercard will get you just about everywhere. Don;t even think about doing traveler's checks (cheques)...they are NOT well received in the USA.
If you're planning on camping in bear country, you could consider getting some bear pepper spray. I always feel more comfortable having something with me for those nights where you know there can be some activity. Don't keep any food/fruit or strong cosmetics in your tent.
Old Baldy screwed with this post 08-13-2010 at 10:39 PM