A concave mirror and a convex lens are held separately in water. What changes do you expect in their focal lengths??

Focal length of any mirror (concave) does not depend on medium, so it remains unchanged. 
If refractive index of the material of convex lens is greater than that of water, then its focal length will increase.
If refractive index of the material of convex lens is equal to that of water, then its focal length will become infinity.
If refractive index of the material of convex lens is less than that of water, then its focal length will become negative i.e it will behave like concave lens.