Idaho has a "seaport" already, in Lewiston (across the border from Clarkston WA). You get there by way of the Columbia River and locks. I often haul giant rolls of paper in my semi from the papermill in Lewiston.
Also by way of the Snake river (which gets to the Columbia just before the tri cities). The snake river goes all the Jackson hole, but isn’t navigable for most of that (well, those parts are very good for white water rafting).