They would probably make sense as a level 0 rail road. Canals became obsolete as railways took off, so players should have the option to build them before they invent railways.
I wonder if improvements can be made 'obsolete' like military units could be in Vicky 1. That way once you have railways, canals could get obsoleted and you could skip straight to building level 1 railways.
Good idea, but I doubt it will be in Vanilla Ticky. Might be a good idea for a mod though; the Ticky Infrastructure Mod (TIM).