In the usual sense, depth is measured at the centerline, amidships. To be truly specific, some reference should be given (such as Benson did above, or on a lines drawing I am looking at that states "Depth Amidship - Inside -..."
I do it by placing the end of the rule on the plank closest to amidship (sometimes there is a rib dead-amidships), and sight across the two gunwales to get the depth. Add 1/4" (+/-) to get the depth to outside of hull.
I do not usually include the keel when reporting depth. (I would include the keel in reporting "draft," which is the minimum amount of water depth required to float the canoe).
Dan