The wavelength changes.
Work Step by Step
The particles in the 2 mediums physically interact. If the air molecules vibrate up and down at frequency f, then the water molecules must vibrate at the same frequency, because otherwise they would not and could not stay in contact with each other to pass the wave from air to water. Sound travels about four times faster in water than in air. We note that the wave speed increases, while the frequency does not change, so the wavelength must increase. $$v = \lambda f$$ The sound’s wavelength in water is about four times larger than it is in air.