I think you are confusing coefficient of drag with friction coefficients.
The Cd for a boat hull is dependent on Reynolds number and Froude number. Surface conditions do have an effect as long as the flow is fully turbulent. However, the effect of surface roughness on drag greatly depends on the flow situation i.e. of a blunt body vs. a streamlined body. In a streamlined case, the surface roughness tends to increase drag (airplane wings need to be very smooth). In blunt bodies, the surface roughness tends to decrease the drag (golf ball). I would think a boat hull is a streamlined body, so that means that you'll have to have a reference for your geometry at varying roughnesses to properly account for that aspect.
For boat hulls the Froude number [tex]Fr = \frac{U}{\sqrt{lg}}[/tex] comes into play, but I'm not a marine designer, so I can't say for sure how much dependence there is on it. The amount of drag produced due to a wave formed at a two fluid boundary is a second drag factor you need to account for. I would estimate that it is as important as the reynolds number though if the speed was high enough. One book I have has a chart for a particular boat hull (with and without a bow bulb) and it's Cdw vs. Fr. I would think it would be your best bet to see if you could get something like that from the manufacturer of your inflatable.
I would say that unless you want to do some model testing, which is normally what is done, you'll have to take a swag and estimate the geometry using known geometries in tables of known Cd's and then apply a fudge factor for the wave effects.
I don't think there's any easy way to get somewhat reliable numbers here. Perhaps someone who has access can model it for you and run some CFD code on it.
P.S. You just need to know the pounds of thrust required, not ft-lbs.[/size]