Do not drink the local water. Bottled water seems to be cheap enough, but sometimes hard to find for a good price. The usual soft drinks called sucré in Congo such as Coke, Pepsi, Um Bongo and Mirinda are available in most places and are safe to drink. Local drinks like Vitalo are amazing. Traditional drinks like ginger are also common.
The local beer is based on rice, and tastes quite good. It comes in 75 cl bottles. Primus, Skol, Castel are the most common brands. Tembo, Doppel are the dark local beers.
In rural areas, you may try the local palm wine, an alcoholic beverage from the sap of the palm tree. It is tapped right from the tree, and begins fermenting immediately after collection. After two hours, fermentation yields an aromatic wine of up to 4% alcohol content, mildly intoxicating and sweet. The wine may be allowed to ferment longer, up to a day, to yield a stronger, more sour and acidic taste, which some people prefer.
Beware of the local gin. Sometimes unscrupulous vendors mix in methanol which is toxic and can cause blindness. Some people believe that the methanol is a by product of regular fermentation. This is not the case as regular fermentation can not yield methanol in toxic amounts.