A snake, starting from its neck, thrusts its body by contracting its muscles. Thus it creates a series of curves. But without scales it will be very difficult for snake to move.
The moment of snakes is also known as "Undulatory locomotion".
With the help of these scales it grips the ground and contracts it's muscles and moves the body sideways. In this process the body appears in a wave like motion resembling 'S'.
In water the thrust given by the muscular contraction and sideways motion propels it forward.
In water there is no gripping surface so the movement is dependent on muscular power.
Knowledge is power.