A water taxi, maybe I could see an argument for, but a ferry? Where would it be docked? It also is quite expensive to maintain and uses a lot of fuel. Also would the ferry fit under all the bridges (I have no idea if any/how many of the bridges are draw bridges). It makes no sense to have a ferry on the river. Ferries run in Seattle in the depths of the Puget Sound and travel to other islands and they carry vehicles.

Because only one side of the river wants it, and north-south boat traffic would gum up an East-West river. Just ask the Portland Spirit.