KALININGRAD, Russia — The maritime museum in this Russian exclave on the Baltic Sea caps each summer with its international Water Assembly, an antic parade of small historical vessels from around the Baltics, their crews wearing period costumes as they sail the Pregolya River.

But this year, said Svetlana G. Sivkova, the founding director of Kaliningrad’s Museum of the World Ocean, regular participants from neighboring Lithuania and Poland threatened to stay home.

“They said they could not come to us because Poles and Lithuanians are being beaten on the streets of Kaliningrad,” said Ms. Sivkova, appalled at what she called an abrupt and unwarranted change in mood.

“These are intelligent, educated people,” she added. “It’s horrible propaganda. We had to explain that it’s not true, that we are an open people.”