> Leeches freak me out, I can't imagine swimming with (or falling on) the gators!
FWIW, neither leeches nor alligators are indigenous to salt water, which is what surrounds the Florida keys.
The easiest way to discern each is based on their snout. If the reptile you saw has a blunt nose about the same width as its jaw, then it's a gator. If, instead, the jaw looked more like a trapezoid, then it's a croc.
Both are opportunistic hunters capable of taking down mammals up to adult bovines or horses. The latter two examples are rare as the size of the croc/gator has to be rather large.