While I see how this suggestion can quickly lead to these kind of other ideas and conclusions let me add something.
The only goal I had in mind when making this suggestion was to provide another way to explore the mainland, this time by using rivers instead of roads. We’re talking rafts here, after all. A quite inappropriate and vulnerable navigation method.
In my mind, just the river itself could and should kill you if you’re not careful, if things such as rapids were implemented. But even without that we have giant flying swamp creatures that could drain you dry in a couple seconds. If anything, navigating a raft would be something quite risky. Its main benefit would be offering you an alternative (relatively) fast transportation method which is (relatively) easy to build, for those not versed in mechanics or not lucky when looking for a terrestrial vehicle, or parts for it, or gas, etc.
It’s also something that goes well with the “survivor” theme of this game.