KAZAN, Russia — The rain was not relenting on Sunday afternoon as the Bulgaria left the riverbanks for the open expanse of the reservoir. So the baby sitters gathered the children together and led them to the activity room to play.

When the cruise boat lurched to one side and then slowly capsized, the room became a sealed container. Adults broke windows with the heels of their shoes, or slid down the corridor and scrambled into the water. Meanwhile, baby sitters managed to fasten life jackets on some of the children — there were 40 children, by some estimates. But they could not break out of the playroom.

On Monday, divers were bringing the dead to the surface one by one. In all, 128 people were presumed drowned. It was the worst loss of life among children in Russia since 2004, when terrorists seized a school in the south. Russian television showed rescued parents standing on a pier, screaming. “The child was left behind,” one man sobbed, wrapping his arms around a woman. “The child was left behind.”

As many as 208 people were on board when the boat sank, including at least 59 children. About 80 people were hauled out of the water about an hour and a half later, when the third of three passing vessels stopped to help.