There are certain areas of the hull where it is not recommended to use core material and these include the skeg, keel and areas below the cabin sole line, effectively, anywhere below the waterline so, in cored boats, these areas of the mould should be sheeted with a cheap polyurethane foam which is later removed. The keel sides and bottom can be covered with ¼” [6mm] Hardboard/Masonite which is waxed and a release agent applied to eases the removal of the mould former from the hull. Some keel moulds have to be made detachable from the hull mould to facilitate removal of the hull.  It would be wasteful to install an  expensive core materials in areas where it will later be removed. In our own designs we always recommend that the area under the cabin sole be single skin so this means that the hull area below the waterline should always be covered with a cheaper foam which is shovelled out after the hull has been turned upright.   As with all forms of construction, there are many ways to achieve the same results.   You should follow your plans.   If you have a good idea not covered in your plans, then consult your designer before making any major changes to the recommended building methods.