I don't think posters were advocating that you put it down whatever over water.
Thank you HT , that was exactly my point and you have saved me inviting MAS to go back and read my post again. I don't recall any, never mind most ppruners advocating immediate ditching. In fact I was precisely avoiding that word because this could happen over land as well and still not be within reach of an airport and I always combined smoke/fire to try and indicate it could be both or it could be either or. Failed obviously.
So to reiterate: In
some (the worst) circumstances, if you are not within shouting distance of a surface you could run out of options.
HT, you quite correctly bring in another possible problem about insufficient fuel, if you do descend and have also successfully dealt with the problem during that descent. However, if you do not give yourself the option, then all that extra fuel at altitude may be of no use to you whatever.
I don't suggest that there is an easy answer to this, because there may not be but I do strongly feel its worthy of discussion on many flights.
Question: In this particular instance, what were the US Airways crew's options if they had not succeeded in dealing with the smoke?