My guess would be black racer, by the speed you describe. I would like to rule out Eastern indigo but they are known to feed on fish. was the snake thick bodied or slender? If its an indigo leave it alone as its illegal to even hold them in florida. Eastern Coachwhips from down south can be dark in the front half but tend to lighten to a khaki color the rest of the way.
Get a pic!
Nate