Englishal
I take your point about setting 7700 but the fix is only as good as the UHF range between radar head and transponder, when the transponder is only 30 feet above the water that is not very far.
A radial and distance from a GPS transmitted from 30 feet and relayed from an airliner at FL370 can be put into an FMC (it takes eight FMC key strokes to get a direct track to a R/D position) and put a rescue helicopter within 100M of the ditching site.
The whole idea is to get the rescue helicopter to the crash site ASAP and before you die in the cold water, should you be unfortunate not to get into your liferaft.