So, the drag due to a submarine just below the surface will be more than the drag due to a submarine deeply submerged, because just below the surface say for example, this is the water surface and a submarine is here floating in water, then the frictional resistance is due to this surface and wave making, now you put the submarine down below here, just below the water surface you have the full frictional resistance through the full surface and also you have wave making resistance on top, whereas, if the submarine is down below, very much down below, it has only the frictional resistance and not the wave making resistance at all, so this case will be the worst case in the case of a submarine.