How do buoyant forces affect free fall?
If buoyant forces are present, the object is not in free fall.
The definition of free fall requires that the only force on the falling body is gravitational force, its weight. Consider a solid metal object tossed into water. It will have an upward buoyant force in addition to its gravitational force, preventing us from saying it is in free fall.
The acceleration of a falling object is given by Newton's 2nd Law:
Net force is the vector sum of all forces acting on a body. Therefore the acceleration of our solid metal object decreases when it is in the water because the buoyant force is upward -- opposite the direction of gravity's force on it. (There will also be another upward force, frictional drag due to the whatever speed it develops going through the water.)
I hope this helps,