Why does the Cessna 208 Amphibian have extra tail surfaces?
Well, it's the same reason that the Challenger ultralight demonstrates extreme &quot;adverse yaw&quot; 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.)  &quot;Directional&quot; 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.
Which begs the question-- Why did the Wright brothers take steps to increase the amount of vertical surface area forward of the CG on some of their aircraft?