Like milestones....except in water.
Docks would be place that someone travelling along a path would enter as the natural automated part of travelling on the route. It would automatically convert a cow and a wagon into a raft with the cow and the wagon assuming you had a raft in the wagon. The dock itself would function as a buoy/milestone and find it's way along rivers from buoy to buoy like milestones. And then to dock to reverse the function and continue on land.