I've seen the same thing happen when the floats are setup such that the wing has too much incidence angle when the floats are planed out. The plane is trying to level out which drives the front of the floats down into the water. You should have the tops of the floats about parallel with the bottom of the wing. Also, your floats should be around 44 inches long. Do you have any float ahead of the prop? There should be a couple inches..........Seaplane