MYSTIC, Conn. — A national historic landmark slowly slid into the water on Sunday, to cannon fire and the cheers of thousands of spectators on land and in boats.

After nearly five years, about $7 million and a painstaking restoration by more than 60 people, the Charles W. Morgan, believed to be the last surviving wooden whaling vessel in the world, was again afloat — 172 years after its construction.

“Once it’s floating, it’s alive again,” said Quentin Snediker, the director of the shipyard at the Mystic Seaport museum, who was in charge of the restoration....