Many trainer type (flat bottom wing) airplanes have as much as 5 or 6 degrees of downthrust to compensate for the wings tendancy to create higher lift as the speed increases. This downthrust will pull the nose down and cause the airplane to dive momentarily if high throttle is added at low airspeeds.

To verify this, trim the average trainer to fly level at low speed, do a low, slow flyby and then hit the throttle as it passes in front of you. You will see the airplane dive before it picks up enough speed to balance the downthrust with extra lift.

Now when you advance the throttle on these same airplanes on floats, the downthrust will try to push the nose into the water until the wing starts to make enough lift (and the stab starts to make enough negative lift) to bring the airplane back into equilibrium.

Drag is a major factor as well, so don't be surprised if your floats dig in.

Jim