Grainger - make a safe emergency landing and swim for it if the donkey failed?
If he (or she..) was flying mid-point vertically (s)he would have been 50' above the water. That doesnt give you a lot of time to set yourself up for a successful auto IMO. And if s(he) had been just approaching the bridge, then a zoom climb to trade speed for height would have been out of the question.
So contrary to what you said about the number of options available, there would be only one - dump, flare, check, level - splash. And it would probably happen as quick as you can read it.
However, if you are going to do a stunt like this, you probably weren't going to consider the donkey stopping anyway, so its all a bit academic really, I suppose!