“The seas are quite bad,” said Elijah J. Waterman, a spokesman for the American Embassy in Malta. “They’re just holding in place right now.”

The United States sent the seagoing ferry — a tourist vessel with flat-screen televisions and a small casino — on Wednesday to transport about 600 people, mostly Americans, after being turned down for permission to land a chartered plane in Tripoli. The passengers would remain on the ferry, which has been secured, and were being provided with food and water, Mr. Waterman said.