Giving docks a Portage ability similar to the Commandery’s Deploy ability could be a workaround to this using existing mechanics instead of trying to change terrain types like my initial proposal. It could let any units garrisoned in a dock deploy to any other dock. You’d still need to build a dock for each separate body of water, but it would at least mean your fishing boats wouldn’t be stuck in a puddle after all the fish are gone. It would be even more useful than my original suggestion because it would also allow movement between bodies of water on maps like Yucatan or Malaysia. I believe the bigger canoes can also garrison in docks so that might need to be reevaluated if they could also teleport.