In every civilization since ancient times people tended to settle as close to the water, the base of living, as possible. And big rivers were some of the best places to make settlements which could further grow and develop into cities because of great possibilities for trade. This statement is quite correct concerning the Volga, a 3530 kilometers long river, one of the longest rivers in the world and the longest one in Europe. Thus many important Russian cities are located on it, of which four have population over 1.000.000: (down the river) Nizhny Novgorod, Kazan, Samara, and Volgograd.
The region has historically been multi-ethnic, and despite some of the nations and tribes have eventually assimilated or completely disappeared, there are still representatives of several nations of which most notable are Russians, Tatars and (in the southern part) Kalmyks. There are also small groups of Volga Germans still remaining from the XVIII century when they were invited to live there by Empress Catherine the Great, despite the fact that in 1941 many of them were deported into remote parts of the USSR by the government as they were considered potential Nazi collaborators. But despite ethnic diversity, Russian is lingua franca like in any other part of Russia and is understood by absolutely everyone. And most of younger people are also able to communicate in English.
River cruises down the Volga operate during the summer months (early May to end of September). Dozens of boats operated by different companies run from Moscow to Astrakhan. One way or return cruises may be reserved to/from practically any city along the Volga. Turflot.ru and infoflot.ru are several sites that offer tours.
Sample price for cruise: Moscow to Astrakhan (one-way) with three daily meals is RUR 15,000 (approximately 500USD). Without meals, approximately RUR 8-9,000.