You may have found the answer. I've found that when a float or both floats, when planed out, are riding to close to the nose, the plane will want to veer to one side or the other. It's similar to a tail dragger with the gear too far foreward....very hard to control. This can be caused by improper setup or some down elevator trim. You want the floats riding close to the step, with everything behind the step up out of the water......Seaplane