public void setRefractionConstant(float refractionConstant) {
        this.refractionConstant = refractionConstant;
        if (material != null) {
            material.setFloat("R0", refractionConstant);
        }
    }
This is a constant related to the index of refraction (IOR) used to compute the fresnel term.
F = R0 + (1-R0)( 1 - N.V)^5
where F is the fresnel term, R0 the constant, N the normal vector and V the view vector.
It depends on the substance you are looking through (here water).
Default value is 0.5
In practice, the lowest the value and the less the reflection can be seen on water

@param refractionConstant the desired R0 value (default=0.5)
jme3-effects/src/main/java/com/jme3/water/WaterFilter.java