I've seen extra tail surfaces that look like additional vertical stabilizers on the C208 Amphibian. Why is there a need for this? I can't imagine that the floats reduce vertical stab effectiveness.
Well, it's the same reason that the Challenger ultralight demonstrates extreme "adverse yaw" in response to aileron inputs when flown with the doors on, but less so when flow with the doors off. The floats-- like the doors-- increase the surface area in front of the CG, which has an effect similar to decreasing the size of the vertical fin (which, naturally, is behind the CG.) "Directional" stability is decreased.
Just like if you took a weathervane and added enough surface area in front of the pivot point, it would stop working.
It's not uncommon for aircraft on floats to have modifications to increase the effective area of the vertical fin, usually by adding a small ventral fin below the fuselage.