Acoustic based Remote

How hard would it be to build a simple remote based on acoustics rather than radio waves?

What I have in mind goes like this, transmitter sends out acoustic waves via submerged transducer which is pick up by a receiver on the sub.

Something like what the single channel remotes of old but use an arduino to encode and decode the signals.

1 beep - forward
2 beeps- stop
3 beeps- turn left
4 beeps- center
5 beeps- turn right
6 bèeps- dive planes up
7 beeps- dive planes neutral
8 beeps- dive planes down
9 beeps- ballast vent
10 beeps- ballast blow

two ideas come to mind from the 'olden days' of electronics, one is the acoustic modems that used to warble when loading the programmes in an old sinclair spectrum type PC, the other is that before infra-red and radio based TV remotes there were a number that used ulttrasonic accoustic signals, both would provide a number of digital on/off channels.
Trouble is I doubt if the chips that they used are still made and getting microphones/speakers to work underwater would be a challenge, though there were systems that used the coil from a 'speaker agaisnt a thin steel sheet that then vibrated. That would also work as a microphone, though not sure how sensitive it would be. Hi-Fi quality is not needed however, so a unit could be made waterproof, as long as the depth wasn't too deep or there was a pressure equalisation method say with another rubber diaphram that tookup the change in volume due to pressure.
Ultrasonic transducers would be more tricky, though you could pinch some ideas from the pingers talked about here.
The ideal might be to get hold of the transducers from a boat echo sounder which would work at ultrasonics, but might be too directional.

Keep it relatively simple.
20mhz ought to have the range, though that range depends on the power of the initial sound.
As your looking at a reasonably simple set of commands a complex system with sound modulation is probably not what you need, a simple click is going to be as effective and esier to achieve.
The sender could be a piezo crystal transducer BUT these tend to be low power (not enough range) and the signal is usually directional so maybe a thing as simple as a speaker might work. Though speakers are designed to push air, water is denser so the ammount of power needed would be much higher.

It would be nice and simple if you could use a fish finder sender which would manage the range easily (100m+ arc from the sender) but the arc would be limiting. You could track the sub with the fish finder?? and have the sender point in the right direction ???

A small but rather significant issue would be that the signal you send, a modulated warble or a click, will interact with the surroundings and bounce around.
Imagine you have a solid reflecting wall or object somewhere in the water you are using. Your signal goes off through the water to your receiver, you pick it up and decode the signal into an action. So far so good but here comes a problem, the signal has bounced off an object in the water somewhere and your receiver gets a second signal. Discriminating between the two might be a problem
Also if the reflecting object was close to the receiver it isn't beyond the realms of probability that your CLICK CLICK (or warble) becomes CLICK CLICK CLICK CLICK.
Using your scheme instead of stopping to avoid ramming your mates sub, you centre the rudder to ensure a nice dead centre hit ;)

Do able but with some potential issues