the water taxi on Bangkok's main river, the Chao Praya is more of a ferry than a taxi. they work like a subway where they slam into the dock, unload and load, take off ... all of which takes just a few seconds. there's a set fee 15 B [30 cents US] one way and you can just pay for your ticket on the boat.
more like a taxi are the long tail boats. they are mostly around the popular tourists docks and fees need to be negotiated. they also decorate the prow.