Scubanero,
I think you are on the right track to solving your problems. I've found some of the trainer type planes have loads of down thrust in the engine which pulls the front of the floats down when excellerating. I think the down thrust was put in to help prevent the plane from nosing up during excellerating due to the drag of the wing being high above the thrust line. When you add floats to a high wing plane you also add drag below the fuselage, so in theory you shouldn't need the down thrust in the engine any more. This is just my opinion and I have no proof to back it up.
Seaplane