In CMG Walker I wrote about how to make a mobile machine with a totally rigid and seamless surface: no joints, no wheels, no flexion.
A glider or submarine of this sort could perhaps work exclusively by moving its center of mass internally, like the OrbSwarm robots, rather than using reaction wheels or CMGs; this is a significant part of how hang-gliders work. In the case of a submarine, a similar tactic can alter its buoyancy, which can be used to propel it forward, though this requires either a flexible surface or some transport of mass, probably water, across the surface, for example through a hole.