The Kok River boat trip is Thaton's classic onward journey: a longtail run east through hill country toward Chiang Rai. When the water is right, it turns a transfer into a half-day of river bends, village stops, forested banks, and glimpses of the borderlands.
Do not assume it runs like a scheduled city ferry. Low water, rain, and lack of passengers can all change the plan, so ask in town before building an itinerary around the boat.