RE: Once on step plane whips around and dunks
The floats have too much incidence. At liftoff speed your fuselage goes horizontal and the plane rides up on the nose of the floats-way ahead of the Cg. You have more incidence in the wing than you realize...incidence is not the bottom of the airfoil, it's measured from the most-forward point of the leading edge, to the trailing edge. You have plenty of reserve incidence to become airborne. Meanwhile the plane is acting like a grocery cart pushed backward.
Rework the front strut and raise the noses of the floats 3/4 to 1"