Hi Dave976. the American Beauty model has 6 rudders. It is not at the simple end of making model boats work. So taking everything on board the only thing to do is to accept the situation as it is. One of the accepted methods of getting the rudders to function is to use the pair of conventional stern rudders linked together and mixed with the 2 esc's for the throttle.
The right stick rudder control actually activates the mixing with the left up/down stick controlling speed and direction.
I believe you are an ex-flyer so perhaps not used to going backwards. So in addition and separate from the esc's and the RH rudder servo you can fit the forward flanking rudders to a separate servo and link all 4 together and all turning in the same direction.
When the LH Tx lever is operated it does not operate the mixing only the direction and speed so the stern rudders stay where they are.
However when going in reverse with the LH stick depressed (this does not activate the mixing) it is easy to introduce left and right movement to operate the second servo operating the 4 linked rudders thereby giving simple steering in reverse.
The model has a complex rudder system with 3 rudders per prop with a Kort nozzle in between. So simple is not an option, however the system above does make it easy to work as only one hand is needed at a time. With tank control both hands are needed which can be a nuisance.
There are at least 3 other ways of connecting the rudders and I have connected up the rudders on my American Beauty model in the method outlined above. The main reason I use this method is that the flow of water into the Kort nozzle is not deflected by unwanted rudder movements in the inlet water.
Many Transmitters have mixers now and the LH stick lateral movement is often redundant so connecting a second rudder servo not connected to the mixing works out well. When operating the left / right movement on the throttle stick you will not be using the right hand stick at all.
If using a mixer operating from the receiver I would check that full servo movement is available on each channel i.e. not shared between them.