For me choice of bulkhead (shadow) material depends on the displacement of the finished model.
If you are going to end up adding lead ballast to bring the model down to water line you can afford to use thicker stuff like 3/8 or 1/2 ply, use good quality 2 good face exterior ply if you can and check for voids in the laminations as these can cause weaknesses in your frames.
The good thing with thicker bulkheads is that you have more gluing surface and somewhere to hang the planks with brass pins (or steel pins if you sink the heads into the planks and stop up the holes).
If weight is a premium I would go with 1/8 or 1/4 birch ply or thicker grades of hard balsa making 1" ring frames.
Plywood is OK for keels but your local builders merchant or timber mercant will buzz up a length of stick from some exotic hardwood used in door frames etc for just a few quid for exposed keels or ply is good if the keel is covered by planking ie as in a WW2 motor torpedo boat construction.
I have used both of the methods above in my current Master Hand Build found here on the forum
For planking material you can get away with cheap pine but try to find stock that allows you to cut planks out quarter sawn (annular rings running front face of plank to back face of plank) as planks cut this way are stronger and are less prone to crushing and denting during collisions in the finished model
Choose pine boards free of knots of course.
I use this method to find planed finished boards and take them home and buzz them up into plank sizes on my cheap table top circular saw fitted with a tungsten tipped circular saw blade.
Daubing the interior of the hull with glass fibre resin and finishing the exterior of the hull in resin and fibreglass tissue will keep out the elements for the Sunday outings to the local pond as model boats are rarely kept on permanant moorings so this method of waterproofing will prove sufficient