What is the magnitude of buoyant froce acting on a 720 cm^3 toy submarine foating in fresh water? (d water=1 g/cm^3)
That will depend upon whether the submarine is full submerged or not. It can float without being fully submerged.
If it is fully submerged and not resting on the bottom, the buoyant force is the weight of displaced water, 0.72 kg*g = 7.06 N