Jimmy Buffett had a similar accident off of either Long Island or Nantucket. He was acceleating for take off and one of his outer pontoons got hit by a large wave. The Pontoon submerged and flipped the plane over. Luckily for him there was no fire and he was able to escape.
Interesting theory..the plane in question is a high wing design..and it was taking off froma fairly sheltered area..there was no wave actrion..had they somehow clipped a wavetop, due to a wing dip....the pilot would have aborted the take off..
If that were the case, the plane would have been so low to the water that it would have crashed immediately. There would be no part falling from the sky after tthe plane. I am guessing that it was metal fatigue compounded by salt water corrosion.