Question

A concave mirror is placed in water what would be the change in the focal length of the mirror

Answer 1

there is no change in focal length if we keep in water also

Answer 2

the laws of reflection is not affected by the surrounding medium. Therefore the focal length of concave mirror doesn't change when it is immersed in water, though in case of lenses the focusing property of is dependend on the nature (refractive index) of the surrounding medium.