Thanks Glenn, excellent answer with more than enough detail!!

Thanks also for the suggestion of trying the solid cypress although I
would preferably like to finish my Water Wheels with a stained design
so should I apply the stain after the epoxy and primer?

Finally (I promise, last question), I give customers the option of a
wooden (Marine Plywood treated with epoxy) or galvanised steel stand
although the stainless stell stand is very expensive, time consuming
and difficult to manufacture. In your own opinion, do you think I can
use Marine plywood extensively for the Water Wheel stand even though it
will be permanently partly-immersed in water? I also want the option of
being able to secure the Wheel at a specific height on the stand which
I reckon I can master.