Rippentuck came to a great idea - lets make fast/swift water-travel. It seems feasible, and more or less lore-appropriate. This could be used either for swift-travel between points between distant locations (using only simple loading screens) or as a fast-travel trip (much like mounts are now) so that players could, if they choose, simply float down these rivers and watch the sights go by on the way to their destinations.
We hope Turbine will also like this idea. Berephon has pointed out that:
From a lore perspective, there is nothing preventing river travel and would certainly become more reasonable once we start extensively expanding on water-bounded territories (but I wouldn't predict it being anything other than swift travel).
As far as a pathing boat or boat-mounts, as Scenario mentioned quite a while back, it would be very resource intensive to actually make happen.