What is the probability that passengers can survive a ditching in a 747 at MTOW?
Aircraft are designed with ditching in mind, but the outcome depends on many circumstances, most importantly on how calm the water is and how well the aircraft can be controlled.

The Wikipedia article on Ditching lists emergency water landings and other accidents where aircraft ended in water.

There is no ditching by 747, so we don't have any specific reference. However:


B747 is not that different from other types of aircraft. Controlled ditchings on calm water were successful in different types; it is likely that one would be possible with B747 too.
How long the aircraft would remain afloat depends on the damage. But if it did not break up, it would likely manage to keep afloat for the 90 seconds needed for evacuation. And the slides can be used as life rafts.
There was one case where B747 ended up in water after overrunning runway (so at lower speed than landing), China Airlines flight 605, and the aircraft did remain afloat long enough to evacuate it.
Also any aircraft that will be used for flying across large bodies of water, which obviously includes all variants of B747, must be certified for ditching. This involves designing it so that, at least according to model, a controlled ditching will be smooth enough not to cause serious injuries to occupants and will leave it afloat long enough for successful evacuation.