Instead of having your map in the canoe bilge, get a clip that snaps onto round tubing, and screw a binder clip to it (the
W.H.A.C.K.O. site ought to have a picture of this, but it doesn't seem to at the moment). Anyway, you snap it onto a thwart, and clip your map with it, which keeps it out of the sump, and in a place where it's a bit more readable.
I have a slightly more elaborate rotatable gizmo (homemade) that I put the map in, and I then attach the gizmo to the thwart with this clip thing. I can also hang the gizmo around my neck with clip leads if need be, and I have binder clips on the handlebars of my bike (attached with zip-ties) so I can use it while riding.