Ferries are a convenient way of providing transportation across the river. They need not be placed directly across from each other, and their use does not impede other river traffic.
Let's say you want to establish a small community across the river to collect reeds. To get the immigrants across the river in the first place, all you need do is simply place the ferry depots.
The immigrants do not need roads and will provide their own boats. At this point, the depots do not need roads or staff.
When you are ready to establish regular service, you must provide roads and depot staff. Once the ferries are running they will transport directional walkers. All others will be turned back.