I always thought that one of the objectives of SG was to stop the legs from sinking. To this end I would reach forward quite aggressively, with hands breaking the surface and an unnatural arch in my back. I was doing just that recently on a Coach Certificaton Course, when Terry steped in and corrected me. He said its fine if the legs sink. The important bit is to maintain good posture. He encouraged me to drop the arms deeper, the resulting effect being that the scapular are pulled flat onto the back.
Just to reiterate, floating the legs to the surface should not be an objective in SG, and certainly does not indicate skill level. Some people's legs just float better than others.