Cruises on the Gambia River are very popular and can be arranged in Banjul, as well as in most of the larger hotels throughout the Gambia. The trips usually take three days and involve a busride to Georgetown, where the boat is boarded that sails down the river to Banjul, or vice versa. Town along the Gambia River that are usually visited include Kuntaur (and the Wasu Stone Circles), Kudang, Tendaba Camp, Jufureh (Juffure) and James Island, although not all cruises visit all of these places.
Usually the Gambia River Cruises are over-advertised, as three day cruises (while actually it is two days + one day in the bus), while adverts suggest you'll see a lot of wildlife. In reality all you might see are some monkeys, several hippos and dolphins, if you're lucky. Most likely you will sit on the ship for 2 days and all you'll see are mangrove trees.
The Gambia River Cruises are overprized and not worth the money.