Great Toradog, always precise and punctual.
Lower frequencies correspond to longer waves and, vice versa, higher frequencies correspond to shorter waves.
The longer the waves, the more they are able to overcome obstacles.
On the other hand, increasingly smaller electromagnetic waves will interfere (and be "slowed down") by increasingly smaller elements, until reaching the extreme cases of the electromagnetic spectrum (made up of X-rays and Gamma rays that interfere with molecules and atoms). But here we are well beyond radio frequency.
Recalling in broad terms the operating principle of the microwave oven, I think that microwaves should be avoided when attempting to penetrate this fluid. The frequency of 2.4 Ghz is very close to the resonance frequency of water.
This is what I have always known from physics books.
All this in theory but I needed practical confirmation like that of Toradog.
Thank you very much for your reply Toradog.

"Would I be able to use a standalone mixer on the rudder and throttle,"
Short answer is yes . It should do the job nicely.
The computer in the 2.4ghz radio is the other half of what makes them so attractive to folks these days, me included.
Sadly subs and other underwater craft suffer by the loss of that technology.
There are a couple of gents, here across the Pond, who have experimented and succeeded in using 900mhz. I am using an old Robbe unit at 75mhz and the range and underwater reception is excellent.

Hi
I have sent you a D/M
Fred

Many thanks Toradog, that’s really helpful. I think a few of our ‘older’ members may still have 40mhz kit 😉
Would I be able to use a standalone mixer on the rudder and throttle, to power two thrusters? I’m used to a modern 2.4 GHz TX that can mix on the radio… thanks again

Eric, Thee rule is that the lower the frequency, the better the penetration in H2O. Here in the States, it means that the old 27 and 75mhz radios worked best for subs. That said, I think that you are restricted to 40mhz? as your old frequency and 2.4 ghz as the old range. 900mhz is an option on both sides of the Pond, I think. If you can find an older 40mhz set, you will be better off, as it will be more reliable than even 900mhz.
Hope this helps