Bartica sits where the Mazaruni and Cuyuni meet the Essequibo, and everything here happens on the water. Day tours from Georgetown come up the river and take in Fort Island, Kyk-over-al and the falls; locally there are island resorts, and the working boat service that crosses the estuary all day.
If you want the river without an itinerary, the speed-boat charters are the way to do it.