Sounds like the floats weren't mounted straight... If the floats are on crooked, you can get all kinds of wierd control problems, and poor aileron or rudder response is very common when the floats aren't straight.

Also... some planes need the rudder and fin enlarged when you put floats on them. The added profile (what you see by looking at a side view of the aircraft) area forward of the CG requires an increase in the profile aft of the CG.