Visas are not available on arrival, and showing up without one can cause many things you will want to avoid at all costs fines, passport confiscation, etc.
For tourist visas you need to obtain an invitation letter from Congo and print and take that with a copy of your plane tickets to the nearest Republic of Congo Embassy, with a usual processing time of 3 days for $100.
CTT Congo Travel and Tours can arrange a letter and tourist visa within 1-2 days. Mikhael's hotel in Brazzaville, Ledger Plaza Maya Maya, and other expensive but excellent 5-star hotels can also arrange an invitation letter upon booking a room.
Maya-Maya Airport IATA: BZV ICAO: FCBB in Brazzaville is linked by flights to by Air France, Douala in Cameroon, Addis Ababa and Kinshasa by Ethiopian Airlines, Nairobi and Casablanca. More routes and foreign carriers are under negotiation.
Pointe Noire PNR has additional connections including some truly luxurious private airlines such as Swiss-run PrivatAir, in addition to a host of the large commercial companies and a few domestic Congolese airlines ferrying over to Brazzaville.
The brand new Chinese-built BZV airport looks like a super-modern emerald city from the wizard of Oz. Kinshasa's is so run down and horrible looking that you almost laugh at the juxtaposition. Your entrance is usually smooth and comfortable and the airport is secure. Make sure you have all your documents in hand. Lines are not long and officials smile generally, though a well-placed bribe of even $5 will make you speed through VIP style.
The airport information desk in the arrivals terminal is friendly, multilingual, and excellent, and has on hand tourist information and safe taxis and transportation options. For quick questions the US embassy is almost in walking distance from the airport.
Small but safe motorized or traditional boats can be arranged through tour agencies in the Congo. They include life jackets and petrol and run between Brazzaville and scenic riverside villages, hotels, and restaurants overlooking the river and rapids. Mami Wata has large boats for 12-15 people, and CTT www.congotravelandtours.com has boats for 1-10 people for day trips or 5-7 day river voyages to many locations and scenery along the Congo River. The ride is generally very safe and the wildlife and riverlife is spectacular.
Passenger and VIP ferries operate daily between Brazzaville and Kinshasa roughly every 2 hours between 8AM and 3PM. Prices for the ferries are: US$15 for the passenger and US$30 for the VIP ferry. The VIP ferry is recommended as these are brand new boats and are not as cramped. A valid visa for both countries is required in either direction. The bureaucracy at either end require some time. Entry and exit procedures in Brazzaville are "easy" and straight forward and people are very helpful in assisting to get through without troubles. In contrast, these procedures are a bit difficult in Kinshasa and depend much on whether you are an individual traveller or assisted by an organisation or an official government representative. There are also speed boats to hire, either in a group or alone price!, however, it is not advisable to book them as they really speed across the river along the rapids. Barges follow the Congo, then the Oubangui, rivers right up to Bangui.
It's safe to drive in the Republic of the Congo. A good sealed road goes north from Brazzaville, but only as far north as President Sassou's hometown of Oyo. Beyond Oyo, the roads get very bumpy and are totally impassable in the rain. It is also very hard to get a rental car you drive yourself.
As of 2013 the road between Brazzaville and Pointe Noire is excellent, and the road connecting Pointe Noire and Cabinda is also excellent. SUV's can be purchased in Pointe Noire for half to a third of their selling price in Brazzaville, and resold there.
In direct competition with the road between Brazzaville and Pointe Noire for tourists, there is a new jungle train that leaves twice a week from the capital and takes 8-13 hours to reach the coast. You can book the train as part of a tour in Congo to enjoy the stunning scenery from first class cabin, or show up a week in advance to haggle for your ticket and seat berth if you're more flexible on time and cash.