DQ,
I think HC is hinting to the fact that with a MGB Chip light, and vibrations (and other related warnings indicating an imminent failure of the gearbox) it would be wiser (if survival was your primary goal) to carry out a controlled ditching, pop the floats and man the liferafts - very well demonstrated by a Bond EC225 crew earlier this year.
Rather than....
Flying along slow and low, 'limping' home - During this 'limping home' stage if the MGB were to fail catastrophically, I doubt there would be much time to do much, let alone pop the floats. The change in attitude of the aircraft, I'm guessing, would be dramatic.