Why wont the water wave undergo a phase change of pi when propagate to a fixed wall.?

I suppose you mean water waves REFLECTED on a wall.

Water molecules close to the wall are free to move. As such, the situation is similar to the reflection of transverse wave on a string with free end. No phase change will be occurred in both cases.

A phase change of pi radians occurs only when the end of the string, where the wave reflects, is fixed.