The ship went down in a huge reservoir near the city of Kazan on Sunday.
Officials said dozens of people are missing. Rescuers have found 79 survivors.
More than 200 people were aboard the Bulgaria when it sank. Officials said it was licensed to carry only 120 people.
Authorities are investigating the official cause of the disaster. Survivors said the ship was leaning as it made a turn and a wave washed over the deck, the state news agency RIA Novosti quoted an official as saying. It sank within about eight minutes.
The agency cited local investigators as saying the ship was listing even when the voyage began, possibly because of unemptied sewage tanks, and that the port engine was malfunctioning.
The ship sank about two miles from shore in about 65 feet of water, officials said.
President Dmitry Medvedev on Monday demanded a thorough investigation and declared Tuesday a day of mourning. He also called for a full technical assessment of the condition of all Russia's passenger vessels.
The Associated Press contributed to this report.