I wonder if where the main flow has been canalised we shouldn’t tag them as waterway=river with something like canalised=yes or canal=yes. For navigation purposes boat=* should still provide necessary information. Note that there are other issues which I find unsatisfactory with waterway=canal (mainly use both for irrigation and navigation canals): particularly for small irrigation channels which may be no more than 50cm wide.
Note neither canal or canal_type seem to be in use to indicate this type of canalisation either.