Imagine a sound wave, longitudinal of course, with a really low frequency, say 1 cycle per second, 1Hz. Now imagine this wave beating down on the surface of water... one beat per second, like a drum... boom, boom, boom,... etc. The 1Hz beat is put in the water. It is still the same frequency, 1Hz.

Without getting into temperature and pressure and all sorts of other things that affect the speed of sound, let's just be simple and say that the speed of sound in air is 343 m/s and in seawater it is 1560 m/s, but to make the math super easy, lets just round these speeds to 300 m/s and 1500 m/s respectively. That makes the speed in seawater 5 times faster.

Every time a wave front travelling at 300m/s in the air hits the surface of the water and enters the water, it immediately speeds up and shoots through the water at 1500m/s.

This diagram is rotated 90º CCL.

Stated again: A wave front hits the surface of the water once per second (1Hz sound wave). That 1Hz wave enters the water, is now travelling much faster, but was still generated by a 1Hz disturbance. It's as if a drummer was hitting the surface of the water once per second. The sound wave produced in the water moves at 1500m/s, but the frequency of this drum beat is the same.

Another analogy: Imagine a line of cars all going in a single straight line down a road. They are all spaced equally, like 10 feet between them. All of them slavishly follow the speed limit regardless of their spacing, so if the lead car were to slow down all of the sudden, there would be a major pile up. But the lead car doesn't slow down. All of the cars are going 10mph as the signs on the road indicate. Then, a new speed limit sign... 50mph. As each car passes this sign, it speeds up from 10 to 50mph. Clearly, as each car passes the new sign, it will speed away from the car behind it until the car behind it passes the sign and then it can also go 50mph. The new formation will be a line of cars, but they will no longer be spaced 10 feet apart, they will be spaced further apart. Just how far apart they are after the speed change can be figured out, but it look to be a tedious operation since I gave the spacing in feet and the speeds in mph... meaning a bunch of calculator work... boring. But this shows the same situation as the sound wave: velocity, frequency, and wavelength.