There is acutally a lot of breeding facilites down there it probably did come from one, chanes are it probably came from one that doesnt have a permit. A lot of people here own things illegally/un-documented and or without a permit in the state, its just sad they don't consider the effects from their bad keeping practices. There is no established breeding population (yet, that would be the really scary part).
As someone who is out on the water a lot all around florida it really is unsettling, I would hate to be in my yak or out fishing somewhere and then one appears. I've encountered an american croc before while I was out surfing, very shy though and kept a distance (of course I kept a distance and made my way back to shore). They don't really have the bad rep like a nile does. A lot more shy and much much smaller.
As for the nile/american crocs hybridizing I don't really see that happening I think the species are two different, but you never know. I don't believe the caimans we have had hybrdized with anything, not sure if its possible.