Hi Nicole!
They're not exactly the same. A fully submerged object implies that the object is completely immersed IN the fluid but ABOVE the bottom of the basin. A sunk object is completely immersed in the fluid but sitting on the bottom of the basin.
The main difference between the two is the presence/absence of the normal force from the bottom of the basin.
With a fully submerged object, there is NO normal force. Instead, Fg = Fb.
With a sunk object, there IS a normal force. So, Fg = Fb + N.
(where Fg is gravitational force, Fb is buoyancy force, and N is normal force)