i had an idea for canals which required spending a GE to construct them, and they take so many turns to build. i intended that idea as a sort of canal wonder such as panama or suez, rather than the network of canals in england say which basically were superceded by railways after a fairly brief time in historical terms. i also thot the GE should be able to build tunnels in same way like the channel tunnel, or superbridges which i cant think offhand of any names but i am sure some bridges exist that ships can still travel beneath but which span channels of water. those kinds of 'wonder improvements' are all industrial age or later. i have read somewhere about ancient canals in greece and egypt that existed tho, and some in china. leaving them out entirely from civ seems an oversight - and using the fort graphic is less than satisfactory. maybe should have a choice which to build, if the hex has sea on two opposite sides. then you choose fort if its just a headland or canal if its two coastlines such as in panama on earth map.
i stress my suggestion of a specific graphic choice for canal which would be less defensible than a fort, because of the importance of canals in history. as someone said war was fought over suez.