How does the frequency and wavelength of sound change during the transition from air to water?

Water is a much denser medium than air, so vibrations from one source (in particular, sound) will propagate at different speeds.

In less dense environments, the wavelength will be longer than in denser environments. Perturbation of the medium (transmission of the resulting densification through molecular bonds) will be transmitted longer (roughly – the distance between the molecules is much greater, therefore, the transfer of energy between them is slowed down in time).

Thus, if a sound source (say, a membrane of sufficient size) is half immersed in water, then the wavelength when sound propagates in water will be shorter than its wavelength in air for the reasons specified above (the frequency, in this case, will be determined by the source sound and therefore will not change).