At least 21 people were killed on the Nile north of Cairo when a cargo ship hit a party boat late on Wednesday night.
Five of the party were rescued from the river but at least six were still missing early on Thursday.
Security sources said the search for them had been hampered by the darkness.
At least two children were killed. Family and friends of a young couple had hired the boat to celebrate their engagement. It was not clear whether the couple were among the dead.
Police said they had arrested the captain of the cargo boat and his deputy after the accident, one of many that happen on the Nile and off Egypt's coast each year.
The deadliest example happened in February 2006, when an Egyptian ferry sank in the Red Sea, killing more than 1,000 people.