Barking, what you describe is what happens if you did'nt correct the wing/float incidence from the way the kit is designed. You can check other threads on the KMP Beaver on how to remedy this but the bottom line is that you have to get the floats adjusted so they are 2-3 degrees negative to the mean chord line of the wing.
If the plane sits quite tail down in the water you have an incidence problem. Proper set up is essential on the Beaver and it appears KMP didn't do their homework on this one.
Peter