My best guess is that it is to avoid having a nose up attitude causing a sharp pitchdown as the aircraft impacts the water, as that might damage the spongy components strapped into the seats?
Haha, LOL!
I think the emphasis is on
not putting the plane in a
dynamically changing attitude, because of the unpredictability of the water surface.
Trying to stall it in would be fine if you have a flat calm surface and you can actually pull it off - but if there's any swell or chop the water level might fall away from underneath you just at the critical moment, such that your profile is changed to a state much worse than desired - in big enough swell the a/c could go post stall and dump you in the drink quite visciously! However, in a steady descent with the aircraft in a stable attitude, the final ditch will be pretty consitent regardless of swell (unless you're really unlucky and ditching into very rough sea).
Also what looks flat and calm at 200 ft might look completely different at 2 ft.
Any seaplane drivers out there with more to offer?