When a sound wave passes from one medium to another, such as from air into water, its frequency is unchanged. The frequency is set by the source of the sound, and the number of waves arriving at the boundary each second must equal the number leaving it.
The speed does change. Sound travels faster in denser, stiffer materials: about 340 m/s in air, 1500 m/s in water and around 5000 m/s in steel. It travels fastest in a solid and slowest in a gas.
Wave speed = frequency × wavelength. If the frequency is constant, wave speed and wavelength are directly proportional. When a sound wave speeds up, the wavelength becomes longer by the same factor. A 500 Hz sound has a wavelength of 340 ÷ 500 = 0.68 m in air, but in water it has a wavelength of 1500 ÷ 500 = 3.0 m. If the wave meets the boundary at an angle, it also changes direction, which is called refraction.