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.
By ship
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.
By plane
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.
By car
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