I will use an acrylic cylinder (3mm wall thickness) as the watertight section and make the end caps using a 3d printer. Probably ABS or PETG with a O ring seal.
I will try to get working planes as well as the rudder(s) and lock them in place initially. I think that I may have to depart from scale for the rudders and aft planes as there is very little clearance and support in the stern. Possibly a single large rudder and move the planes up to a wider part of the hull to get a space with more room for support and a servo arm. Or a servo arm outside the keel through a hull bottom slot to the planes.
From reading on here I think that I will use bellows for the servo rods, though so far I haven't discovered where they come from. One of my old aircraft modelling friends said that he used to run in buggy motors in water. I know that a brushless motor will (attempt) to run in water (overturned floatplane) so I wonder if anyone on this forum has experience of running the motor in the wet section? Otherwise I suppose its another sealing problem round the prop shaft. As most surface running boats don't seem to have a leak problem I assume that standard shaft tube seal reasonably well.