andy ,
i think you are wroung about the wheel digging a hole , after all, wasnt the revs quoted at 45 rpm?
at that speed you need all the thrust you can generate from the wheel , more , not less floats .
one idea that would solve the problem , would be to fit 2+555 motors w 50mm props , under the stern, the wash from these would disapear in the wheel wash, these would be recessed into half tunnels in the bottom hull.
there is nothing worse than running a model that you have spent yrs on in construction , that crawls across the lake , and wont steer