The 'Ferry Boat'
Like most cities with rivers in Thailand, Bangkok's has regular ferries taking people from one side of its main artery, the Chao Praya River, to the other. Of course in the south, where there's extensive travel between the mainland and Thailand's islands, ferries are bigger affairs, carrying passengers and cars.
Costs certainly vary - a trip across a river could be as low a cost as 1 or 2 Baht; a journey to one of the far off islands, obviously, much more.