As for the Canoe Builder's Hut, I specifically did not want ti to upgrade into the Shipwright because it can be used for Rivers. Big ships cannot enter rivers which is why canoes can still be used (until steamboats).
Just to point out - if Canoe Builder's Hut were to upgrade to Shipwright, but Shipwright requires Coastal Access, you could still build Canoe Builder's Hut on cities that are river only. I think what we need to figure out is OR building prerequisites on units so you don't have to stay locked into one building to build a given unit. Then your Shipyard could also build Canoes for your river squares (even though they currently can't be built once you get War Galley).