Another factor may have been that they were departing over a downward slope.
This can make staying low in the cushion more difficult as there can be a tendency to want to climb rather than descend to stay in the cushion. Note that they seem to maintain altitude rather than a constant height which would be required to stay in the cushion.
I'm guessing that by the time they realised it wasn't going to work the option of s safe reject was past and they were committed to the water.
I also would not be surprised if there turned out to be a tailwind component.
As for the comments about the floats not inflating - they won't inflate if they have not been armed. Perhaps they did not arm them because they considered that this was an overland departure rather than over water?
At least they all survived to tell the tale