500 feet, he should have noticed that the right side floats were only partially inflated and climbed to a safe altitude and flown off to land elsewhere.
Is that tongue in cheek or have you not been following what happened?
As for inflating the floats at 800 ft, that’s about 15-20 seconds before ditching and if - as he said - he was committed to landing in the river it seems a good decision. Inflating lower would just mean landing a few more meters further up the river!