Actually, the drag of the mains pitched him over; the nose gear just
finished the job.

It's interesting to speculate if he would have flipped if it was just
the mains. Of course they all go up and down together. No one really
knows for sure, but I tend to think it wouldn't be a full flip if it
werent for the nose gear catching. No one knows for sure. There HAVE
been gear up water landings where there wasn't a flip, but generally,
landing with the gear down in the water is pretty bad news. Landing
with the gear UP on land is not so bad, no prop strike, by the way.

Gear switch on an amphib is a little different mindset. You don't just
turn the warning off by putting the gear down. You position the gear
where it should be and turn the warning off by pushing a button. Also,
the amount of drag presented by amphib gear is not as dramatic as gear
on land planes, so its not always obvious where the gear is by wind
noise and how the plane flies.