its possible we do get canals as a tile improvement. If one of the wonders visually adds a canal to the map (this is what firaxis wrote wasnt it?), then maybe we can also build them with workers.
I do hope they will be hard to build, kinda like in SMAC with those boreholes and humidity super improvements. If it's too easy, then we will see cities way into the mainland with a connection to the sea all over the place, just to be able to crank out sea units.
A road could easily be bridged over it (science:bridgebuilding?), and sea units could navigate through them.
A good rule maybe for using canals is that it has to connect two water bodies, and the building of one would be automated. Somewhat how you could use teraformers (also in SMAC) to "build road to".
Allowing cities bordering canals the ability to make naval units, i'm not too sure about, it might be a problem. And terrain bonus, for sure give bonus to trade and food too! A canal is great help for irrigation I thought...
So having canals, it might finally allow those cities on inland seas to actually be able to build naval units and have them be put to use (excluding carriers, which were cool inland-sea-airbases with civ games..).
