Question

What happens to concave lens when dipped in water considering refractive index of lens is less than that of water?

Answer 1

only thing I would add is that the extent of the change in focal length depends on the curvature of the lens surface that is in contact with the water. A camera for use underwater can use a lens in contact with water at its front face but air on the inside. If the front surface is flat, there will be little change in focal length whether immersed in water or air.