Most places such as cities have potable water, so don't worry about drinking tap water. Bottled water is also available at low prices. Nevertheless, exercise caution about places with doubtful water sources. These places tend to be away from population centers or near the beach/coast.
Refrescos or simply frescos are beverages made from fresh fruit cas, guayaba/guava, sandia/watermelon, mora/blackberry, fresa/strawberry, piña/pineapple, papaya, sugar and either water or milk depending on the fruit. All sodas serve these. You can also easily buy the standard international soda pops; 'Fresca', 'Canada Dry' and the local 'Fanta Kolita' fruit punch are recommended.
Typical "refrescos" include: Agua de Cebada Barley water and Horchata de arroz.
A common drink is called "Cacique", which is made from fermented sugar cane. It is sometimes called "guaro", which can also refer to alcohol in general. It is similar to vodka, and is usually drunk with water and lemon. It is very inexpensive, however note that it's not a very "clean" liquor, so exercise caution.
There are approximately 8 different national beers available and most international, which are sold in cans, bottles and even kegs. The most common beers in the country are Pilsen and Imperial: all bars and restaurants serve both. Bavaria, "Bavaria Negra" dark and Bavaria Light are considered higher quality but more expensive, Rock Ice and Rock Ice Limón lemon flavor has a higher alcohol percentage. Heineken is locally made under license and is more expensive as well.
Coffee in Costa Rica is of very top quality. It is common to have coffee, not only for breakfast, but during the afternoon as a little coffee break.
A typical drink here is called Aguadulce, which is a hot drink made of water and Panela unrefined whole cane sugar. Aguadulce literally means "sweet-water".