Answer
Longer.
Work Step by Step
Water’s index of refraction does not differ as much as air's does from the glass lens's index of refraction. There is less refraction when light passes from water to glass and vice versa than when light passes from air to glass and back out.
In other words, the light’s path will not change as much when the lens is immersed, so its focal length will be longer when it is in water.