  This paper considers two-dimensional steady continuous stratified periodic
water waves. Firstly, we prove that each streamline must be symmetric about the
crest line when it is strictly monotonous between troughs and crests by
exploiting the maximum principle and analysis of surface profile. Then,
standard Schauder estimates are exploited on the uniform oblique derivative
problems to show that all streamlines are real analytic (including the free
surface). Based on above symmetry and regularity of streamlines, finally we
provide an analytic expansion method to recover the water waves from horizontal
velocity on the axis of symmetry and wave height. Most notably, all of results
here are suitable not only for small amplitude but also for large amplitude.
