I'm not sure whether this is intentional, but all boats ignore waterway restrictions and base the ability to navigate canals on their weight.
exe: Simutrans-Experimental_160922_41d2457 & the pakset was the latest from the server as of the 09.09.16
Thank you for the report: this seems to have been caused by the way constraint of only the first vehicle (the horses in this case) being checked. I think that it is fixed now. Are you able to re-test to confirm?