This cruise across the lake is a very scenic way to make your way from Chungju towards Danyang via Janghoe (or vice versa). The cruises can get very busy on weekends and in high season. There’s a prerecorded sightseeing commentary (Korean only), so it's not the most relaxing experience, though the placid scenery is beautiful.
There are numerous routes but the most popular cruise (fast boat 1½ hours, ferry 2¼ hours) is from Chungju Dam to Janghoe via Cheongpung (and in reverse); the rocky cliffs are most dramatic between the later stops. Ferries are irregular in the low season but pretty much leave every hour in summer.